Prenatal Ultrasound Guidelines

Introduction to Prenatal Ultrasound

Prenatal ultrasound is a crucial diagnostic tool for monitoring fetal development and detecting potential complications during pregnancy, as recommended by the American College of Radiology and the American College of Obstetricians and Gynecologists 1, 2, 3

The American College of Radiology recommends optimal timing for most accurate dating using crown-rump length (CRL) at 7-12 weeks 2
The yolk sac is visible at approximately 5.5 weeks, and an embryo with cardiac activity is typically visible by 6-7 weeks, according to the American College of Radiology 2
The first scan should be performed as early as possible to determine chorionicity and amnionicity, ideally in the first trimester, as recommended by the American College of Obstetricians and Gynecologists 3
Transvaginal ultrasound can detect a gestational sac at approximately 4.5-5 weeks, and the discriminatory level of hCG is approximately 3000 mIU/mL, as stated by the American College of Radiology 1, 4

Ultrasound performed between 11 weeks 0 days and 13 weeks 6 days includes nuchal translucency measurement, assessment of nasal bone, basic fetal anatomy survey, and maternal serum markers (if combined screening is performed) 2

The American College of Obstetricians and Gynecologists recommends follow-up scans every 2 weeks starting at 16 weeks for monochorionic-diamniotic twins to monitor for twin-twin transfusion syndrome 3

Women with risk factors for fetal growth restriction should have additional growth scans in the third trimester 5
Women with a history of preeclampsia should have additional ultrasound with fetal biometry, amniotic fluid assessment, and Doppler studies when preeclampsia is diagnosed 6

Ectopic pregnancies can develop and be detected up to approximately 16 weeks gestation, though most are diagnosed between 6-10 weeks when symptoms typically appear, with the most common locations including tubal ectopic pregnancy (84-93%) 7
Sonographic findings typically include an extraovarian mass or adnexal mass separate from the ovary, tubal ring, and in some cases, visualization of an extrauterine gestational sac with yolk sac or embryo, as reported by the American College of Radiology and the American Journal of Obstetrics and Gynecology 7, 4
Definitive diagnosis requires visualization of an extrauterine gestational sac with yolk sac/embryo (100% specific), or a tubal ring or extraovarian mass with no intrauterine pregnancy, according to the American College of Radiology and the American Journal of Obstetrics and Gynecology 7, 4

Definitive criteria for diagnosis of early pregnancy loss (EPL) include: embryonic crown-rump length (CRL) ≥7 mm with no cardiac activity, mean sac diameter (MSD) ≥25 mm with no embryo, and absence of embryo with cardiac activity ≥14 days after visualization of gestational sac with no yolk sac, as stated by the American College of Obstetricians and Gynecologists and the Society of Radiologists in Ultrasound (SRU) 7, 2
Concerning criteria for EPL include: CRL <7 mm with no cardiac activity, MSD 16-24 mm with no embryo, and absence of embryo with cardiac activity 7-13 days after visualizing gestational sac with no yolk sac, as recommended by the SRU 7, 2

The management approach depends on patient stability, location of the ectopic pregnancy, gestational age, presence of cardiac activity, and β-hCG levels, with options including medical management with methotrexate, surgical management, and expectant management (in select cases) 7
Three management approaches can be offered: expectant management, medical management, and surgical management, with the choice depending on patient preferences and clinical circumstances, as suggested by the American College of Radiology and the American College of Obstetricians and Gynecologists (ACOG), with a strength of evidence level of high 4, 2, 7
Follow-up ultrasound is needed to confirm complete expulsion for expectant or medical management, and hCG levels should be monitored until they return to normal 8
The SRU and the American College of Obstetricians and Gynecologists advise against premature diagnosis of EPL without meeting established criteria, and against immediate intervention before confirming diagnosis with follow-up ultrasound, to prevent misdiagnosis and unnecessary treatment 7, 2

Heterotopic pregnancy, a rare coexistence of intrauterine and ectopic pregnancies, is more common in assisted reproduction, as noted by the American College of Radiology and the American Journal of Obstetrics and Gynecology 7, 4
Enhanced myometrial vascularity (EMV) can be confused with arteriovenous fistula or malformation, leading to unnecessary workup, as noted by the SRU and the American College of Radiology 7, 2
Intracavitary fluid may mimic a gestational sac, but can be distinguished by its shape, contents, or location, as stated by the American College of Radiology and Radiology 4, 2