When a pregnant patient presents with vaginal bleeding in the first trimester, the two conditions that need to be ruled out immediately are ectopic pregnancy and abortion. Threatened abortion is common and occurs in about 25% of clinically apparent pregnancies. Half of the women presenting with threatened abortion ultimately abort, in which cases the embryo is most often already dead. All non-viable gestations eventually abort spontaneously; however, the process of expulsion may be delayed for weeks. The clinical outcome of the process of abortion depends on the status of the cervical os and fetal viability. In such a scenario, determination of fetal viability becomes necessary. Although, clinical history and serial serum B-HCG measurements can provide an insight into the diagnosis, it is recommended that first trimester ultrasound be performed for the assessment of fetal viability in cases of threatened abortion. In addition to signs of fetal life on sonography, subchorionic bleeding is an important factor affecting the outcome of gestations in patients with clinical threatened abortion [Goldstein, et al].
The most common cause for first trimester spontaneous abortion is fetal chromosomal abnormalities. According to McGahan, et al the sonographic diagnosis of early pregnancy failure depends on the stage of development:
1.Stage A- loss at 3-4 weeks of menstrual age, is usually subclinical loss; no sonographic evidence [these are the women who bleed at the time of their period and often never know they are pregnant].
2.Stage B- loss at 5-6 weeks of menstrual age, sonographic signs based on gestational sac findings.
3.Stage C – loss at 7-8 weeks of menstrual age, sonographic signs of embryonic demise based on demonstration of an abnormal embryo or gestational sac.
4.Stage D – loss at 9-12 weeks of menstrual age, sonographic signs of embryonic demise based on demonstration of abnormal embryo. Structural abnormalities of head, heart, etc may be seen.
So, depending on the menstrual age at presentation, the respective sonographic abnormalities may be looked for.
• Embryonic cardiac activity – demonstration of cardiac motion indicates that the fetus is alive. Failure to visualize cardiac activity must be interpreted with caution. Cardiac activity is present in normal embryos before it can be detected on ultrasound. Studies by Goldstein and Levi, et al have shown that in normal embryos with CRL of 3 mm or less no cardiac activity may be visualized on ultrasound and follow up is suggested. When the CRL reaches 5 mm or more fetal heart motion should be identifiable.
•Abnormal gestational sac features –
-A large diameter of the sac [dimensions vary depending on whether transabdominal or endovaginal scan is performed], without a demonstratable embryo or yolk sac [i.e. sac size suggests that fetus is of gestational age where embryo and yolk sac should be visualized].
-Irregular or distorted shape.
-Low position of the sac in the endometrial canal.
-Thin decidual reaction [< 2 mm].
-Absent double-decidual reaction.
•Other criteria –
?Collapsing, irregularly marginated amnion,
?Presence of amnion in the absence of a visible embryo.
?Yolk sac calcification.
In conclusion, when evaluating an early pregnancy with ultrasound, the gestational age predicted by the sac size must be correlated with that predicted by the crown rump to exclude a discrepancy. In cases with any question, serial B-HCG measurements and follow up sonography are performed to assess for interval growth.