| Anomaly scan | |
|---|---|
| Synonyms | Anatomy scan, second-trimester ultrasound, 20-week ultrasound, level 2 ultrasound |
| Purpose | To evaluate fetal anatomy and size |
The anomaly scan, also sometimes called the anatomy scan, 20-week ultrasound, or level 2 ultrasound, evaluates anatomic structures of the fetus, placenta, and maternal pelvic organs. This scan is an important and common component of routine prenatal care. [1] The function of the ultrasound is to measure the fetus so that growth abnormalities can be recognized quickly later in pregnancy, to assess for congenital malformations and multiple pregnancies, and to plan method of delivery. [2]
This scan is conducted between 18 and 22 weeks' gestation, but most often performed at 19 weeks, as a component of routine prenatal care. Prior to 18 weeks' gestation, the fetal organs may be of insufficient size and development to allow for ultrasound evaluation. Scans performed beyond 22 weeks' gestation may limit the ability to seek pregnancy termination, depending on local legislation. [1]
Two-dimensional (2D) is used to evaluate fetal structures, placenta, and amniotic fluid volume. Maternal pelvic organs are also evaluated. Views are obtained using an abdominal ultrasound probe, but a vaginal ultrasound probe may also be used to evaluate for placenta previa and cervical length. Three-dimensional (3D) ultrasound is not recommended for routine use during anomaly scan, but 3D ultrasound may be used to further evaluate suspected abnormalities in specific fetal features. [3]
A standard anatomy scan typically includes: [4]
Second-trimester ultrasound screening for aneuploidies, such as Edwards syndrome and Patau syndrome, is based on looking for soft markers and some predefined structural abnormalities. Soft markers are variations from normal anatomy, which are more common in aneuploid fetuses compared to euploid ones. These markers are often not clinically significant and do not cause adverse pregnancy outcomes, but used in order to decide whether additional tests, which will be more accurate, are needed. [5] Placental evaluation is vital for planning method of delivery, as attempting natural childbirth can cause fatal amounts of blood loss in the mother if the placenta is covering the cervix (placenta praevia) and significantly increases the risk of stillbirth. Additionally, placental and umbilical cord abnormalities are also associated with certain fetal genetic abnormalities. [3]
| Fetal Conditions [6] | Placental Conditions [3] | Maternal Conditions [3] |
|---|---|---|
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Fetal sex is often noted during the anomaly scan, but can also be detected during the quad scan. However, performing an ultrasound for the sole purpose of determining fetal sex without a medical indication is not recommended. [1]
Although ultrasound is a useful screening tool for fetal anomalies, the sensitivity of this scan is highly variable and potentially detects only 40% of all fetal anomalies. The ability of this scan to detect structural fetal abnormalities depends on a variety of factors. These factors include the severity of the anomaly, the background risk of the study population, gestational age at time of scan, the expertise of the ultrasound technician and interpreting obstetrician, and maternal obesity. [1]
Obstetrics is the field of study concentrated on pregnancy, childbirth and the postpartum period. As a medical specialty, obstetrics is combined with gynecology under the discipline known as obstetrics and gynecology (OB/GYN), which is a surgical field.
The placenta also known as after birth, is a temporary embryonic and later fetal organ that begins developing from the blastocyst shortly after implantation. It plays critical roles in facilitating nutrient, gas and waste exchange between the physically separate maternal and fetal circulations, and is an important endocrine organ, producing hormones that regulate both maternal and fetal physiology during pregnancy. The placenta connects to the fetus via the umbilical cord, and on the opposite aspect to the maternal uterus in a species-dependent manner. In humans, a thin layer of maternal decidual (endometrial) tissue comes away with the placenta when it is expelled from the uterus following birth. Placentas are a defining characteristic of placental mammals, but are also found in marsupials and some non-mammals with varying levels of development.
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.
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.
Nearly every pregnant individual undergoes some type of prenatal testing. There are varying levels of simplicity, invasiveness, risk, and information that correlate to each test. These tests can be as basic as taking the mothers weight (routine) to as complex and invasive as taking internal fetal blood samples for chromosomal analysis (genetic). Routine tests are conducted repeatedly throughout an individual’s pregnancy at prenatal checkups to monitor health and progression of the mother and the baby. Genetic testing is now a common practice offered to pregnant women by their healthcare professional at specific stages of their pregnancy to test for birth defects and chromosomal abnormalities, or the possibility of carrying these genetic traits. Testing capabilities and practices are becoming safer and less invasive as this area of research continues to evolve.
Placenta praevia is when the placenta attaches inside the uterus but in a position near or over the cervical opening. Symptoms include vaginal bleeding in the second half of pregnancy. The bleeding is bright red and tends not to be associated with pain. Complications may include placenta accreta, dangerously low blood pressure, or bleeding after delivery. Complications for the baby may include fetal growth restriction.
Oligohydramnios is a medical condition in pregnancy characterized by a deficiency of amniotic fluid, the fluid that surrounds the fetus in the abdomen, in the amniotic sac. It is typically diagnosed by ultrasound when the amniotic fluid index (AFI) measures less than 5 cm or when the single deepest pocket (SDP) of amniotic fluid measures less than 2 cm. Amniotic fluid is necessary to allow for normal fetal movement, lung development, and cushioning from uterine compression. Low amniotic fluid can be attributed to a maternal, fetal, placental or idiopathic cause and can result in poor fetal outcomes including death. The prognosis of the fetus is dependent on the etiology, gestational age at diagnosis, and the severity of the oligohydramnios.
Placental abruption is when the placenta separates early from the uterus, in other words separates before childbirth. It occurs most commonly around 25 weeks of pregnancy. Symptoms may include vaginal bleeding, lower abdominal pain, and dangerously low blood pressure. Complications for the mother can include disseminated intravascular coagulopathy and kidney failure. Complications for the baby can include fetal distress, low birthweight, preterm delivery, and stillbirth.
Placenta accreta occurs when all or part of the placenta attaches abnormally to the myometrium. Three grades of abnormal placental attachment are defined according to the depth of attachment and invasion into the muscular layers of the uterus:
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.
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.
Vasa praevia is a condition in which fetal blood vessels cross or run near the internal opening of the uterus. These vessels are at risk of rupture when the supporting membranes rupture, as they are unsupported by the umbilical cord or placental tissue.
Placental insufficiency or utero-placental insufficiency is the failure of the placenta to deliver sufficient nutrients to the fetus during pregnancy, and is often a result of insufficient blood flow to the placenta. The term is also sometimes used to designate late decelerations of fetal heart rate as measured by cardiotocography or an NST, even if there is no other evidence of reduced blood flow to the placenta, normal uterine blood flow rate being 600mL/min.
Percutaneous umbilical cord blood sampling (PUBS), also called cordocentesis, fetal blood sampling, or umbilical vein sampling is a diagnostic genetic test that examines blood from the fetal umbilical cord to detect fetal abnormalities. Fetal and maternal blood supply are typically connected in utero with one vein and two arteries to the fetus. The umbilical vein is responsible for delivering oxygen rich blood to the fetus from the mother; the umbilical arteries are responsible for removing oxygen poor blood from the fetus. This allows for the fetus’ tissues to properly perfuse. PUBS provides a means of rapid chromosome analysis and is useful when information cannot be obtained through amniocentesis, chorionic villus sampling, or ultrasound ; this test carries a significant risk of complication and is typically reserved for pregnancies determined to be at high risk for genetic defect. It has been used with mothers with immune thrombocytopenic purpura.
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.
The fetal membranes are the four extraembryonic membranes, associated with the developing embryo, and fetus in humans and other mammals. They are the amnion, chorion, allantois, and yolk sac. The amnion and the chorion are the chorioamniotic membranes that make up the amniotic sac which surrounds and protects the embryo. The fetal membranes are four of six accessory organs developed by the conceptus that are not part of the embryo itself, the other two are the placenta, and the umbilical cord.
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.
Amnioinfusion is a method in which isotonic fluid is instilled into the uterine cavity.
Early pregnancy bleeding refers to vaginal bleeding before 14 weeks of gestational age. If the bleeding is significant, hemorrhagic shock may occur. Concern for shock is increased in those who have loss of consciousness, chest pain, shortness of breath, or shoulder pain.
The Pregnancy Outcome Prediction (POP) Study is a prospective cohort study of 4,512 women who have never given birth, recruited at the Rosie Hospital between January 2008 and July 2012.
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