Gestational age

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In obstetrics, gestational age is a measure of the age of a pregnancy taken from the beginning of the woman's last menstrual period (LMP), [1] or the corresponding age of the gestation as estimated by a more accurate method, if available. Such methods include adding 14 days to a known duration since fertilization (as is possible in in vitro fertilization), or by obstetric ultrasonography. The popularity of using this measure of pregnancy is largely due to convenience: menstruation is usually noticed, while there is generally no convenient way to discern when fertilization or implantation occurred.

Contents

Gestational age is contrasted with fertilization age which takes the date of fertilization as the start date of gestation. There are different approaches to defining the start of a pregnancy. This definition is unusual for saying that women become "pregnant" two weeks before having sex. The definition of pregnancy and the calculation of gestational age are also relevant in the context of the abortion debate and the beginning of human personhood.

Methods

According to American College of Obstetricians and Gynecologists, the main methods to calculate gestational age are: [2]

Gestational age can also be estimated by calculating days from ovulation if it was estimated from related signs or ovulation tests, and adding 14 days by convention. [4]

A more complete listing of methods is given in following table: [5]

Method of estimating gestational age Variability (2 standard deviations) [5]
Days from oocyte retrieval or co-incubation in in vitro fertilisation + 14 days±1 day
Days from estimated ovulation in ovulation induction + 14 days±3 days
Days from artificial insemination + 14 days±3 days
Days from known single sexual intercourse + 14 days±3 days
Days from estimated ovulation by basal body temperature record + 14 days±4 days
First-trimester physical examination±2 weeks
Second-trimester physical examination±4 weeks
Third-trimester physical examination±6 weeks
First-trimester obstetric ultrasonography (crown-rump length)±8% of the estimate
Second-trimester obstetric ultrasonography (head circumference, femur length)±8% of the estimate
Third-trimester obstetric ultrasonography (head circumference, femur length)±8% of the estimate

As a general rule, the official gestational age should be based on the actual beginning of the last menstrual period, unless any of the above methods gives an estimated date that differs more than the variability for the method, in which case the difference cannot probably be explained by that variability alone. [5] For example, if there is a gestational age based on the beginning of the last menstrual period of 9.0 weeks, and a first-trimester obstetric ultrasonography gives an estimated gestational age of 10.0 weeks (with a 2 SD variability of ±8% of the estimate, thereby giving a variability of ±0.8 weeks), the difference of 1.0 weeks between the tests is larger than the 2 SD variability of the ultrasonography estimate, indicating that the gestational age estimated by ultrasonography should be used as the official gestational age. [5]

Once the estimated due date (EDD) is established, it should rarely be changed, as the determination of gestational age is most accurate earlier in the pregnancy. [6]

Assessment of gestational age can be made based on selected head and trunk parameters. [7] Following are diagrams for estimating gestational age from obstetric ultrasound, by various target parameters:

Comparison to fertilization age

The fertilization or conceptional age (also called embryonic age and later fetal age) is the time from the fertilization. It usually occurs within a day of ovulation, which, in turn, occurs on average 14.6 days after the beginning of the preceding menstruation (LMP). [8] There is also considerable variability in this interval, with a 95% prediction interval of the ovulation of 9 to 20 days after menstruation even for an average woman who has a mean LMP-to-ovulation time of 14.6. [9] In a reference group representing all women, the 95% prediction interval of the LMP-to-ovulation is 8.2 to 20.5 days. [8] The actual variability between gestational age as estimated from the beginning of the last menstrual period (without the use of any additional method mentioned in previous section) is substantially larger because of uncertainty which menstrual cycle gave rise to the pregnancy. For example, the menstruation may be scarce enough to give the false appearance that an earlier menstruation gave rise to the pregnancy, potentially giving an estimated gestational age that is approximately one month too large. Also, vaginal bleeding occurs during 15–25% of first trimester pregnancies, [10] and may be mistaken as menstruation, potentially giving an estimated gestational age that is too low.

Uses

Gestational age is used for example for:[ citation needed ]

Timeline of pregnancy by gestational age Pregnancy timeline.png
Timeline of pregnancy by gestational age

Estimation of due date

Distribution of gestational age at childbirth among singleton live births, given both when gestational age is estimated by first trimester ultrasound and directly by last menstrual period Distribution of gestational age at childbirth.jpg
Distribution of gestational age at childbirth among singleton live births, given both when gestational age is estimated by first trimester ultrasound and directly by last menstrual period

The mean pregnancy length has been estimated to be 283.4 days of gestational age as timed from the first day of the last menstrual period and 280.6 days when retrospectively estimated by obstetric ultrasound measurement of the fetal biparietal diameter (BPD) in the second trimester. [12] Other algorithms take into account other variables, such as whether this is the first or subsequent child, the mother's race, age, length of menstrual cycle, and menstrual regularity. In order to have a standard reference point, the normal pregnancy duration is assumed by medical professionals to be 280 days (or 40 weeks) of gestational age. Furthermore, actual childbirth has only a certain probability of occurring within the limits of the estimated due date. A study of singleton live births determined that childbirth has a standard deviation of 14 days when gestational age is estimated by first-trimester ultrasound and 16 days when estimated directly by last menstrual period. [11]

The most common system used among healthcare professionals is Naegele's rule, which estimates the expected date of delivery (EDD) by adding a year, subtracting three months, and adding seven days to the first day of a woman's last menstrual period (LMP) or corresponding date as estimated from other means.

Medical fetal viability

There is no sharp limit of development, gestational age, or weight at which a human fetus automatically becomes viable. [13] According to studies between 2003 and 2005, 20 to 35 percent of babies born at 23 weeks of gestation survive, while 50 to 70 percent of babies born at 24 to 25 weeks, and more than 90 percent born at 26 to 27 weeks, survive. [14] It is rare for a baby weighing less than 500 g (17.6 ounces) to survive. [13] A baby's chances for survival increases 3–4% per day between 23 and 24 weeks of gestation and about 2–3% per day between 24 and 26 weeks of gestation. After 26 weeks the rate of survival increases at a much slower rate because survival is high already. [15] Prognosis depends also on medical protocols on whether to resuscitate and aggressively treat a very premature newborn, or whether to provide only palliative care, in view of the high risk of severe disability of very preterm babies. [16]

Stages in prenatal development, showing viability and point of 50% chance of survival (limit of viability) at bottom. Weeks and months numbered by gestation. Prenatal development table.svg
Stages in prenatal development, showing viability and point of 50% chance of survival (limit of viability) at bottom. Weeks and months numbered by gestation.
Completed weeks of gestation at birth21 and less2223242526273034
Chance of survival [15] <1% [17] 0–10%10–35%40–70%50–80%80–90%>90%>95%>98%

Birth classification

Using gestational age, births can be classified into broad categories:

Gestational Age in WeeksClassification
< 37 0/7Preterm
34 0/7 - 36 6/7Late preterm [18]
37 0/7 - 38 6/7Early Term [19]
39 0/7 - 40 6/7Full Term [19]
41 0/7 - 41 6/7Late Term [19]
> 42 0/7Postterm

Using the LMP (last menstrual period) method, a full-term human pregnancy is considered to be 40 weeks (280 days), though pregnancy lengths between 38 and 42 weeks are considered normal. A fetus born prior to the 37th week of gestation is considered to be preterm. A preterm baby is likely to be premature and consequently faces increased risk of morbidity and mortality. An estimated due date is given by Naegele's rule.

According to the WHO, a preterm birth is defined as "babies born alive before 37 weeks of pregnancy are completed." [20] According to this classification, there are three sub-categories of preterm birth, based on gestational age: extremely preterm (fewer than 28 weeks), very preterm (28 to 32 weeks), moderate to late preterm (32 to 37 weeks). [20] Various jurisdictions may use different classifications.

In classifying perinatal deaths, stillbirths and infant deaths

For most of the 20th century, official definitions of a live birth and infant death in the Soviet Union and Russia differed from common international standards, such as those established by the World Health Organization in the latter part of the century. [21] [22] Babies who were fewer than 28 weeks of gestational age, or weighed fewer than 1000 grams, or fewer than 35 cm in length – even if they showed some sign of life (breathing, heartbeat, voluntary muscle movement) – were classified as "live fetuses" rather than "live births." Only if such newborns survived seven days (168 hours) were they then classified as live births. If, however, they died within that interval, they were classified as stillbirths. If they survived that interval but died within the first 365 days they were classified as infant deaths.

More recently, thresholds for "fetal death" continue to vary widely internationally, sometimes incorporating weight as well as gestational age. The gestational age for statistical recording of fetal deaths ranges from 16 weeks in Norway, to 20 weeks in the US and Australia, 24 weeks in the UK, and 26 weeks in Italy and Spain. [23] [24] [25]

The WHO defines the perinatal period as "The perinatal period commences at 22 completed weeks (154 days) of gestation and ends seven completed days after birth." [26] Perinatal mortality is the death of fetuses or neonates during the perinatal period. A 2013 study found that "While only a small proportion of births occur before 24 completed weeks of gestation (about 1 per 1000), survival is rare and most of them are either fetal deaths or live births followed by a neonatal death." [23]

Postnatal use

Gestational age (as well as fertilization age) is sometimes used postnatally (after birth) to estimate various risk factors. For example, it is a better predictor than postnatal age for risk of intraventricular hemorrhage in premature babies treated with extracorporeal membrane oxygenation. [27]

Factors affecting pregnancy length

Child's gestational age at birth (pregnancy length) is associated with various likely causal maternal non-genetic factors: stress during pregnancy, [28] age, parity, smoking, infection and inflammation, BMI. Also, preexisting maternal medical conditions with genetic component, e.g., diabetes mellitus type 1, systemic lupus erythematosus, anaemia. Parental ancestral background (race) also plays a role in pregnancy duration. Gestational age at birth is on average shortened by various pregnancy aspects: twin pregnancy, prelabor rupture of (fetal) membranes, pre-eclampsia, eclampsia, intrauterine growth restriction. [29] The ratio between fetal growth rate and uterine size (reflecting uterine distension) is suspected to partially determine the pregnancy length. [30]

Heritability of pregnancy length

Family-based studies showed that gestational age at birth is partially (25–40%) determined by genetic factors. [31]

See also

Related Research Articles

<span class="mw-page-title-main">Gestation</span> Period during the carrying of an embryo

Gestation is the period of development during the carrying of an embryo, and later fetus, inside viviparous animals. It is typical for mammals, but also occurs for some non-mammals. Mammals during pregnancy can have one or more gestations at the same time, for example in a multiple birth.

<span class="mw-page-title-main">Intrauterine growth restriction</span> Medical condition

Intrauterine growth restriction (IUGR), or fetal growth restriction, is the poor growth of a fetus while in the womb during pregnancy. IUGR is defined by clinical features of malnutrition and evidence of reduced growth regardless of an infant's birth weight percentile. The causes of IUGR are broad and may involve maternal, fetal, or placental complications.

<span class="mw-page-title-main">Amniocentesis</span> Sampling of amniotic fluid done mainly to detect fetal chromosomal abnormalities

Amniocentesis is a medical procedure used primarily in the prenatal diagnosis of genetic conditions. It has other uses such as in the assessment of infection and fetal lung maturity. Prenatal diagnostic testing, which includes amniocentesis, is necessary to conclusively diagnose the majority of genetic disorders, with amniocentesis being the gold-standard procedure after 15 weeks' gestation.

<span class="mw-page-title-main">Obstetric ultrasonography</span> Use of medical ultrasonography in pregnancy

Obstetric ultrasonography, or prenatal ultrasound, is the use of medical ultrasonography in pregnancy, in which sound waves are used to create real-time visual images of the developing embryo or fetus in the uterus (womb). The procedure is a standard part of prenatal care in many countries, as it can provide a variety of information about the health of the mother, the timing and progress of the pregnancy, and the health and development of the embryo or fetus.

Fetal viability is the ability of a human fetus to survive outside the uterus.

<span class="mw-page-title-main">Pregnancy</span> Time of offspring development in mothers body

Pregnancy is the time during which one or more offspring develops (gestates) inside a woman's uterus (womb). A multiple pregnancy involves more than one offspring, such as with twins.

Prenatal development includes the development of the embryo and of the fetus during a viviparous animal's gestation. Prenatal development starts with fertilization, in the germinal stage of embryonic development, and continues in fetal development until birth.

<span class="mw-page-title-main">Prelabor rupture of membranes</span> Medical condition

Prelabor rupture of membranes (PROM), previously known as premature rupture of membranes, is breakage of the amniotic sac before the onset of labor. Women usually experience a painless gush or a steady leakage of fluid from the vagina. Complications in the baby may include premature birth, cord compression, and infection. Complications in the mother may include placental abruption and postpartum endometritis.

<span class="mw-page-title-main">Complications of pregnancy</span> Medical condition

Complications of pregnancy are health problems that are related to, or arise during pregnancy. Complications that occur primarily during childbirth are termed obstetric labor complications, and problems that occur primarily after childbirth are termed puerperal disorders. While some complications improve or are fully resolved after pregnancy, some may lead to lasting effects, morbidity, or in the most severe cases, maternal or fetal mortality.

<span class="mw-page-title-main">Human fertilization</span> Union of a human egg and sperm

Human fertilization is the union of an egg and sperm, occurring primarily in the ampulla of the fallopian tube. The result of this union leads to the production of a fertilized egg called a zygote, initiating embryonic development. Scientists discovered the dynamics of human fertilization in the 19th century.

Postterm pregnancy is when a woman has not yet delivered her baby after 42 weeks of gestation, two weeks beyond the typical 40-week duration of pregnancy. Postmature births carry risks for both the mother and the baby, including fetal malnutrition, meconium aspiration syndrome, and stillbirths. After the 42nd week of gestation, the placenta, which supplies the baby with nutrients and oxygen from the mother, starts aging and will eventually fail. Postterm pregnancy is a reason to induce labor.

<span class="mw-page-title-main">Nuchal scan</span> Routine ultrasound done between 11 and 14 weeks pregnancy

A nuchal scan or nuchal translucency (NT) scan/procedure is a sonographic prenatal screening scan (ultrasound) to detect chromosomal abnormalities in a fetus, though altered extracellular matrix composition and limited lymphatic drainage can also be detected.

<span class="mw-page-title-main">Crown-rump length</span> Ultrasound measurement of a foetus

Crown-rump length (CRL) is the measurement of the length of human embryos and fetuses from the top of the head (crown) to the bottom of the buttocks (rump). It is typically determined from ultrasound imagery and can be used to estimate gestational age.

<span class="mw-page-title-main">Velamentous cord insertion</span> Velamentous placenta

Velamentous cord insertion is a complication of pregnancy where the umbilical cord is inserted in the fetal membranes. It is a major cause of antepartum hemorrhage that leads to loss of fetal blood and associated with high perinatal mortality. In normal pregnancies, the umbilical cord inserts into the middle of the placental mass and is completely encased by the amniotic sac. The vessels are hence normally protected by Wharton's jelly, which prevents rupture during pregnancy and labor. In velamentous cord insertion, the vessels of the umbilical cord are improperly inserted in the chorioamniotic membrane, and hence the vessels traverse between the amnion and the chorion towards the placenta. Without Wharton's jelly protecting the vessels, the exposed vessels are susceptible to compression and rupture.

A fetus or foetus is the unborn offspring that develops from an animal embryo. Following embryonic development, the fetal stage of development takes place. In human prenatal development, fetal development begins from the ninth week after fertilization and continues until birth. Prenatal development is a continuum, with no clear defining feature distinguishing an embryo from a fetus. However, a fetus is characterized by the presence of all the major body organs, though they will not yet be fully developed and functional and some not yet situated in their final anatomical location.

The estimated date of delivery (EDD), also known as expected date of confinement, and estimated due date or simply due date, is a term describing the estimated delivery date for a pregnant woman. Normal pregnancies last between 38 and 42 weeks. Children are delivered on their expected due date about 4% of the time.

Post-maturity syndrome refers to the condition of a baby born after a post-term pregnancy, first described by Stewart H. Clifford in 1954. Post-maturity refers to any baby born after 42 weeks gestation, or 294 days past the first day of the mother's last menstrual period. Less than 6 percent of all babies are born after this time. The syndrome develops in about 20% of human pregnancies continuing past the expected delivery date.

<span class="mw-page-title-main">Circumvallate placenta</span> Medical condition

Circumvallate placenta is a rare condition affecting about 1-2% of pregnancies, in which the amnion and chorion fetal membranes essentially "double back" on the fetal side around the edges of the placenta. After delivery, a circumvallate placenta has a thick ring of membranes on its fetal surface. Circumvallate placenta is a placental morphological abnormality associated with increased fetal morbidity and mortality due to the restricted availability of nutrients and oxygen to the developing fetus.

<span class="mw-page-title-main">Prenatal nutrition</span>

Prenatal nutrition addresses nutrient recommendations before and during pregnancy. Nutrition and weight management before and during pregnancy has a profound effect on the development of infants. This is a rather critical time for healthy development since infants rely heavily on maternal stores and nutrient for optimal growth and health outcome later in life.

References

  1. ↑ "Gestational age: MedlinePlus Medical Encyclopedia". medlineplus.gov.
  2. 1 2 Obstetric Data Definitions Issues and Rationale for Change - Gestational Age & Term Archived 2013-11-06 at the Wayback Machine from Patient Safety and Quality Improvement at American Congress of Obstetricians and Gynecologists. Created November 2012.
  3. ↑ Tunon, K.; Eik-Nes, S. H.; Grøttum, P.; Von Düring, V.; Kahn, J. A. (2000). "Gestational age in pregnancies conceived after in vitro fertilization: A comparison between age assessed from oocyte retrieval, crown-rump length and biparietal diameter". Ultrasound in Obstetrics and Gynecology. 15 (1): 41–46. doi: 10.1046/j.1469-0705.2000.00004.x . PMID   10776011. S2CID   20029116.
  4. ↑ Robinson, H. P.; Fleming, J. E. E. (1975). "A Critical Evaluation of Sonar "crown-Rump Length" Measurements". BJOG: An International Journal of Obstetrics and Gynaecology. 82 (9): 702–10. doi:10.1111/j.1471-0528.1975.tb00710.x. PMID   1182090. S2CID   31663686.
  5. 1 2 3 4 A Simple Solution to Dating Discrepancies: The Rule of Eights Hunter, L. A. (2009). "Issues in Pregnancy Dating: Revisiting the Evidence". Journal of Midwifery & Women's Health. 54 (3): 184–190. doi:10.1016/j.jmwh.2008.11.003. PMID   19410210.
  6. ↑ "Committee Opinion No 611". Obstetrics & Gynecology. 124 (4): 863–866. 2014. doi:10.1097/01.AOG.0000454932.15177.be. PMID   25244460.
  7. ↑ Kędzia, Alicja; Woźniak, Jowita; Ziajkiewicz, Marcin; Dudek, Krzysztof; Derkowski, Wojciech (2009). "Foetal age assessment on the basis of selected parameters of head and trunk". The computer-aided scientific research. Vol. 16. Wrocław Scientific Society. p. 241-246. ISBN   978-83-7374-060-0.
  8. 1 2 Geirsson RT (May 1991). "Ultrasound instead of last menstrual period as the basis of gestational age assignment". Ultrasound Obstet Gynecol. 1 (3): 212–9. doi:10.1046/j.1469-0705.1991.01030212.x. PMID   12797075. S2CID   29063110.
  9. ↑ Derived from a standard deviation in this interval of 2.6, as given in: Fehring RJ, Schneider M, Raviele K (2006). "Variability in the phases of the menstrual cycle". J Obstet Gynecol Neonatal Nurs. 35 (3): 376–84. doi:10.1111/j.1552-6909.2006.00051.x. PMID   16700687. S2CID   30317703.
  10. ↑ Snell, BJ (Nov–Dec 2009). "Assessment and management of bleeding in the first trimester of pregnancy". Journal of Midwifery & Women's Health. 54 (6): 483–91. doi:10.1016/j.jmwh.2009.08.007. PMID   19879521.
  11. 1 2 Hoffman, Caroline S.; Messer, Lynne C.; Mendola, Pauline; Savitz, David A.; Herring, Amy H.; Hartmann, Katherine E. (2008). "Comparison of gestational age at birth based on last menstrual period and ultrasound during the first trimester". Paediatric and Perinatal Epidemiology. 22 (6): 587–596. doi:10.1111/j.1365-3016.2008.00965.x. ISSN   0269-5022. PMID   19000297.
  12. ↑ Kieler, H; Axelsson, O; Nilsson, S; Waldenströ, U (1995). "The length of human pregnancy as calculated by ultrasonographic measurement of the fetal biparietal diameter". Ultrasound in Obstetrics and Gynecology. 6 (5): 353–7. doi: 10.1046/j.1469-0705.1995.06050353.x . PMID   8590208. S2CID   39447672.
  13. 1 2 Moore, Keith and Persaud, T. The Developing Human: Clinically Oriented Embryology, p. 103 (Saunders 2003).
  14. ↑ March of Dimes --> Neonatal Death Archived 2014-10-24 at the Wayback Machine Retrieved on November 10, 2014. In turn citing:
  15. 1 2 (). What are the chances that my baby will survive?. [ONLINE] Available at: http://www.spensershope.org/chances_for_survival.htm Archived 2018-08-09 at the Wayback Machine . [Last Accessed 14 November 2012].
  16. ↑ Verlato, Giovanna; Gobber, Daniela; Drago, Donatella; Chiandetti, Lino; Drigo, Paola; Working Group of Intensive Care in the Delivery Room of Extremely Premature Newborns (2016). "Guidelines for Resuscitation in the Delivery Room of Extremely Preterm Infants". Journal of Child Neurology. 19 (1): 31–4. doi:10.1177/088307380401900106011. PMID   15032380. S2CID   20200767.
  17. ↑ "World's most premature baby defies sub-1% survival odds to break record". Guinness World Records . 10 November 2021. Retrieved 2022-03-15.
  18. ↑ Late-Preterm Infants Archived 2012-05-02 at the Wayback Machine ACOG Committee Opinion 404
  19. 1 2 3 Ob-Gyns Redefine Meaning of "Term Pregnancy" Archived 2017-05-03 at the Wayback Machine , from American College of Obstetricians and Gynecologists. October 22, 2013
  20. 1 2 [ full citation needed ]
  21. ↑ Anderson, Barbara A; Silver, Brian D (1986). "Infant Mortality in the Soviet Union: Regional Differences and Measurement Issues". Population and Development Review. 12 (4): 705–38. doi:10.2307/1973432. JSTOR   1973432.
  22. ↑ Anderson, Barbara A.; Silver, Brian D. (1994). "The Geodemography of Infant Mortality in the Soviet Union, 1950-1990". PSC Research Report No. 94-316: 8.
  23. 1 2 Mohangoo, Ashna D; Blondel, Béatrice; Gissler, Mika; Velebil, Petr; MacFarlane, Alison; Zeitlin, Jennifer (2013). "International Comparisons of Fetal and Neonatal Mortality Rates in High-Income Countries: Should Exclusion Thresholds Be Based on Birth Weight or Gestational Age?". PLOS ONE. 8 (5): e64869. Bibcode:2013PLoSO...864869M. doi: 10.1371/journal.pone.0064869 . PMC   3658983 . PMID   23700489.
  24. ↑ Li, Z; Zeki, R; Hilder, L; Sullivan, EA (2012). "Australia's Mothers and Babies 2010". Perinatal statistics series no. 27. Cat. no. PER 57. Australian Institute of Health and Welfare National Perinatal Statistics Unit, Australian Government. Retrieved 4 July 2013.
  25. ↑ Royal College of Obstetricians; Gynaecologists UK (April 2001). "Further Issues Relating to Late Abortion, Fetal Viability and Registration of Births and Deaths". Royal College of Obstetricians and Gynaecologists UK. Archived from the original on 5 November 2013. Retrieved 4 July 2013.
  26. ↑ [ full citation needed ]
  27. ↑ Jobe, Alan H (2004). "Post-conceptional age and IVH in ECMO patients". The Journal of Pediatrics. 145 (2): A2. doi:10.1016/j.jpeds.2004.07.010.
  28. ↑ Dole, N.; Savitz, D. A.; Hertz-Picciotto, I.; Siega-Riz, A. M.; McMahon, M. J.; Buekens, P. (2003-01-01). "Maternal stress and preterm birth". American Journal of Epidemiology. 157 (1): 14–24. doi: 10.1093/aje/kwf176 . ISSN   0002-9262. PMID   12505886.
  29. ↑ Goldenberg, Robert L.; Culhane, Jennifer F.; Iams, Jay D.; Romero, Roberto (2008-01-05). "Epidemiology and causes of preterm birth". Lancet. 371 (9606): 75–84. doi:10.1016/S0140-6736(08)60074-4. ISSN   1474-547X. PMC   7134569 . PMID   18177778.
  30. ↑ Bacelis, Jonas; Juodakis, Julius; Waldorf, Kristina M. Adams; Sengpiel, Verena; Muglia, Louis J.; Zhang, Ge; Jacobsson, Bo (2018-10-01). "Uterine distention as a factor in birth timing: retrospective nationwide cohort study in Sweden". BMJ Open. 8 (10): e022929. doi:10.1136/bmjopen-2018-022929. ISSN   2044-6055. PMC   6252709 . PMID   30385442.
  31. ↑ Clausson, Britt; Lichtenstein, Paul; Cnattingius, Sven (2000). "Genetic influence on birthweight and gestational length determined by studies in offspring of twins". BJOG. 107 (3): 375–81. doi:10.1111/j.1471-0528.2000.tb13234.x. PMID   10740335. S2CID   43470321.
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