What is the role of ultrasound in threatened abortion less than 7 weeks?

Role of Ultrasound in Threatened Abortion Less Than 7 Weeks

Ultrasound should be performed in all patients with threatened abortion less than 7 weeks gestation, regardless of β-hCG level, to exclude ectopic pregnancy and assess pregnancy viability. 1, 2

Primary Indication: Exclude Ectopic Pregnancy

The most critical role of ultrasound in early threatened abortion is to rule out ectopic pregnancy, which occurs in up to 13% of symptomatic emergency department patients—far higher than the general population. 3

Key principle: Never defer ultrasound based on low β-hCG levels, as this is a dangerous practice. 1, 2

• 36% of confirmed ectopic pregnancies present with β-hCG <1,000 mIU/mL, and ultrasound can still detect 86-92% of ectopic pregnancies even at these low levels. 1
• Algorithms that defer ultrasound until β-hCG reaches a "discriminatory threshold" result in mean diagnostic delays of 5.2 days, with some patients showing evidence of rupture at eventual diagnosis. 1
• The American College of Emergency Physicians provides a Level B recommendation: Do not use β-hCG value alone to exclude ectopic pregnancy in patients with indeterminate ultrasound findings. 1

Ultrasound Approach at <7 Weeks Gestation

Transvaginal ultrasound is the preferred modality for evaluating threatened abortion in early pregnancy. 3, 2

What to Look For:

Intrauterine pregnancy confirmation:

• Gestational sac (2-3 mm) may be visible as early as 4.5-5 weeks gestation. 3
• Yolk sac becomes visible at approximately 5.5 weeks and confirms a definite pregnancy. 3, 2
• Cardiac activity typically appears at 6 weeks gestational age. 3, 2

Ectopic pregnancy findings:

• Absence of intrauterine pregnancy with positive pregnancy test is concerning. 1
• Extraovarian adnexal mass or "tubal ring" (fluid center with hyperechoic periphery) suggests ectopic pregnancy. 1
• Free fluid in the pelvis, especially with internal echoes suggesting blood, is highly concerning even without an identifiable mass. 1

Prognostic Value for Pregnancy Viability

Beyond excluding ectopic pregnancy, ultrasound provides valuable prognostic information in threatened abortion:

Fetal heart rate is the most powerful predictor of pregnancy continuation:

• 92% of patients with fetal heart movements at first scan carried to term, while 91% without fetal heart movements aborted. 4
• After 9 weeks gestation, recording of fetal heart movements is 100% reliable. 4
• Embryonic/fetal heart rate >113 beats per minute has 95.3% specificity and 97.2% positive predictive value for pregnancy continuation to 28 weeks. 5

Crown-rump length measurements:

• Fetuses in threatened abortion may show early fetal growth delay (smaller CRL than expected from menstrual age). 6
• This growth delay is a risk factor, as fetuses that ultimately abort have smaller CRL measurements on average. 6

Clinical Outcomes in Threatened Abortion <7 Weeks

Ultrasound findings in clinically diagnosed threatened abortion demonstrate:

• Viable pregnancies in approximately 37% of cases 7
• Embryonic death in 27% 7
• Anembryonic pregnancy in 17% 7
• Ectopic pregnancy in 1.1% 7
• Molar pregnancy in 2.2% 7

Timing and Urgency

For hemodynamically stable patients: Ultrasound should be performed within 12-24 hours if immediate imaging is unavailable. 1

For hemodynamically unstable patients: Immediate ultrasound is mandatory, and these patients require emergent surgical consultation regardless of β-hCG level or ultrasound findings. 1

Bedside ultrasound by emergency physicians may expedite diagnosis when available, with meta-analysis showing 99.3% of ectopic pregnancies have no intrauterine pregnancy on bedside ultrasound. 3, 1

Critical Pitfalls to Avoid

Do not assume low β-hCG means low risk:

• Ectopic pregnancies can present at almost any β-hCG level. 3
• Rupture has been documented at very low β-hCG levels. 3

Sensitivity limitations at <7 weeks:

• When β-hCG <1,500 mIU/mL, ultrasound sensitivity for intrauterine pregnancy is only 33% and for ectopic pregnancy is 25%. 1
• However, this does not justify deferring the scan—it means follow-up is essential when findings are indeterminate. 1, 2

Management after indeterminate ultrasound:

• Obtain specialty consultation or arrange close outpatient follow-up for ALL patients with indeterminate ultrasound. 1
• Never discharge a patient with suspected ectopic pregnancy without ensuring reliable follow-up. 1
• Repeat ultrasound at 1-week to 10-day intervals if initial scan is inconclusive. 5