ABSTRACT: Because practices vary in the frequency and performance of ultrasonography (US) and magnetic resonance imaging (MRI) in pregnancy, this workshop was convened in 2012 to address indications for US and MRI, discuss when and how often these studies should be performed, consider recommendations for optimizing yield and cost-effectiveness, and identify research opportunities.
The benefits of obstetric US include determination of gestational age, fetal number, cardiac activity, and placental localization, as well as diagnosis of major fetal anomalies. First-trimester US (<14 weeks) should include evaluation of the uterus, adnexa, and cul-de-sac. Gestational sac location, presence or absence of a yolk sac or embryo or fetus, and crown-rump length should be recorded along with cardiac activity, embryonic or fetal number, and chorionicity if more than 1 embryo, fetus, or gestational sac is present. For uncertain dating, first-trimester US to measure crown-rump length is recommended. Otherwise, routine first-trimester US for dating is not justified. Offering first-trimester screening for aneuploidy assessment at 11 to 13 + 6 weeks is recommended.
The accuracy of second-trimester dating using US measurements decreases with advancing gestation. At least 1 US study should be offered routinely to all parturients at 18 to 20 weeks for pregnancy dating, evaluation of fetal anatomy and the cervix, and diagnosis of multiple gestation, chorionicity, and abnormal placentation. A targeted examination should be reserved for women with factors that significantly increase her risk for structural fetal anomalies. For US assessment of Down syndrome risk, a systematic protocol should specify which soft markers to include, their definitions, and positive and negative likelihood ratios. Choroid plexus cysts are not associated with Down syndrome, and the risk of trisomy 18 is low with isolated cysts. The presence of an echogenic intracardiac focus or mild renal pyelectasis is unlikely to be of clinical consequence, and further risk adjustment is not required if the patient has already had screening. Shortened humerus and femur lengths are US features of Down syndrome; length can also indicate fetal growth abnormalities or skeletal dysplasia. Nuchal fold thickening (≥6 mm at 15–20 weeks) has a high specificity for aneuploidy, but its association with congenital heart defects is not entirely clear. Echogenic bowel is associated with Down syndrome more than most other soft markers. It is also associated with fetal growth restriction; congenital infection, particularly cytomegalovirus; intra-amniotic bleeding; cystic fibrosis; and gastrointestinal obstruction. An absent or hypoplastic nasal bone is a very sensitive marker for Down syndrome, with a detection rate of 30% to 40% for an absent nasal bone and 60% to 70% for hypoplastic or absent nasal bone.
Specific populations for whom repeat US examinations are warranted include obese parturients, women carrying twins, those with placenta previa or accreta, and those with altered amniotic fluid volume. In obese women, transvaginal US at 12 to 16 weeks improves visualization of fetal anatomy. Determination of chorionicity is preferably done in the first trimester. Twins with monochorionic placentation require heightened scrutiny for twin-twin transfusion syndrome and other unique complications. Ultrasonography every 2 weeks in monochorionic twins should start at 16 weeks and continue until delivery. Doppler US in twins should be reserved for cases where growth restriction is noted or growth discordance of more than 20% is estimated. Doppler US can also evaluate conditions associated with fetal anemia.
Ultrasonography has replaced clinical examination for evaluating suspected placenta previa, which is not a contraindication to vaginal US. If the placental edge is less than 2 cm from the internal cervical os or covering the cervical os in the second trimester, follow-up transvaginal US is recommended at 32 weeks. Earlier US study may be indicated in women who are bleeding. Prior cesarean delivery and placenta previa are risk factors for placenta accreta. Ultrasonography markers of placenta accreta include loss of the normal hypoechoic retroplacental zone between the placenta and uterus, placental vascular lacunae, and placental bulging into the bladder posterior wall. Ultrasonography is the primary tool for diagnosing placenta accreta and can be the only modality used in most patients. Magnetic resonance imaging can be helpful when additional information is needed.
Diagnostic US is considered safe but is a form of energy with effects on tissues and should be used only when clinically indicated, for the shortest amount of time, and with the lowest level of acoustic energy compatible with an accurate diagnosis.
Targeted US must precede fetal MRI, which is not considered a general screening tool and should be used only to answer specific questions raised by US or in specific high-risk situations. Although the research agendas for US and MRI include many aspects of these techniques in relation to fetal and maternal well-being, both modalities currently have a place in the assessment of parturients and their fetuses.