In the United States, approximately 65% of pregnant women have at least one ultrasound examination. Theoptimal timing for a single ultrasound examination in the absence of specific indications for a first-trimester examination is at 16–20 weeks of gestation. Ultrasonography in the firsttrimester may be performed either transabdominally or transvaginally. If a transabdominal examination is not defin-itive, a transvaginal or transperineal examination should be performed whenever possible. First trimester ultrasonog-raphy is used to confirm the presence of an intrauterine pregnancy, estimate gestational age, diagnose and evalu-ate multiple gestations, confirm cardiac activity, and eval-uate pelvic masses or uterine abnormalities (as an adjunct to chorionic villus sampling, embryo transfer, or localiza-tion and removal of intrauterine contraceptives). It is also useful for evaluating vaginal bleeding, suspected ectopicpregnancy, and pelvic pain.
An ultrasound examination may be targeted to help diagnose chromosomal abnormalities in the first trimester. One such examination is measurement of nuchal trans-parency (NT), the lucent area behind the head in the nuchal region. Use of standardized techniques for mea-suring nuchal translucency has resulted in higher detection rates for Down syndrome, trisomy 18, trisomy 13, and Turner syndrome.
Recent studies demonstrate improved detection of Down syndrome at lower false-positive rates when nuchal trans-lucency measurement is combined with biochemical markers (see “Screening Tests” below).
Various types of ultrasound examinations performed during the second or third trimester can be categorized as “standard,” “limited,” or “specialized.” A standard examination is per-formed during the second or third trimester of pregnancy.
It includes an evaluation of fetal presentation, amniotic fluid volume, cardiac activity, placental position, fetal biometry, and an anatomic survey. If technically feasible, the uterus and adnexa also are examined. A limited examination is per-formed when a specific question requires investigation. In an emergency, for example, a limited examination can be performed to evaluate heart activity in a bleeding patient. A detailed or targeted anatomic specialized examination is performed when an anomaly is suspected on the basis of history, biochemical abnormalities or clinical evaluation, or suspicious results from either the limited or standard ultrasound examination. Other specialized examinations might include fetal Doppler, biophysical profile, fetal echocardiography, or additional biometric studies.
Evaluation of placental and cervical abnormalities may be accomplished with ultrasonography. Placental abruption can beidentified by ultrasonography in approximately half of all patients who present with bleeding and do not have pla-centa previa. Color-flow Doppler ultrasound assessment is used to identify placenta accreta. Transvaginal ultrasound examination most accurately can visualize the cervix, and also can be employed to detect or rule out placenta previa as well as an abnormally shortened cervix, which has been correlated with an increased risk of preterm delivery when measured at 26–30 weeks of gestation.