What is a threatened abortion?

Updated: Jun 08, 2018
  • Author: Elizabeth E Puscheck, MD; Chief Editor: Richard Scott Lucidi, MD, FACOG  more...
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Answer

Threatened abortion consists of any vaginal bleeding during early pregnancy without cervical dilatation or change in cervical consistency. Usually, no significant pain exists, although mild cramps may occur. More severe cramps may lead to an inevitable abortion.

Threatened abortion is very common in the first trimester; about 25-30% of all pregnancies have some bleeding during the pregnancy. Less than one half proceed to a complete abortion. On examination, blood or brownish discharge may be present in the vagina. The cervix is not tender, and the cervical os is closed. No fetal tissue or membranes have passed. The ultrasound shows a continuing intrauterine pregnancy. If an ultrasound was not performed previously, it is required at this time to rule out an ectopic pregnancy, which could present similarly. If the uterine cavity is empty on ultrasound, obtaining a human chorionic gonadotropin (hCG) level is necessary to determine if the discriminatory zone has been passed.

The discriminatory zone is the level of hCG beyond which a normal, singleton, intrauterine pregnancy is consistently visible by ultrasound. The discriminatory zone may vary depending on a number of factors, including the hCG assay type and reference calibration standard used, ultrasound equipment resolution, the skill and experience of the sonographer, and patient factors (eg, obesity, leiomyomas, uterine axis, multiple gestations). Also, the discriminatory zone will vary depending on whether the ultrasound is performed abdominally or vaginally. Therefore, having a universal discriminatory zone is difficult, and it optimally should be calculated at each site.

Some studies recommend that a gestational sac should be visualized by 5.5 weeks' gestation; a gestational sac should be visualized with an hCG level of 1500-2400 mIU/mL for transvaginal ultrasound or with an hCG level over 3000 mIU/mL for a transabdominal ultrasound. If the hCG level is higher than the discriminatory zone and no gestational sac is visualized in the uterus, then consider that an ectopic pregnancy may be present. [2] Multiple gestations are an exception and can have higher hCG levels earlier in gestation because more hCG is being made by the trophoblasts from the multiple implantations. Thus, the gestational sac(s) may not be visible on ultrasound despite the hCG levels being higher than the discriminatory zone. Even with multiple gestations, the gestational sacs should be visible at a similar gestational age as singleton gestations or about 6 weeks' gestation if the dating is good.

A clinician should be concerned about ectopic pregnancy but cannot make the diagnosis of ectopic pregnancy just because the hCG level is higher than the discriminatory zone and the uterus appears empty on ultrasound. Many of these pregnancies are abnormal intrauterine pregnancies as opposed to ectopic. One needs to take into consideration the clinical history, and estimated gestational age by LMP or date of conception, if known. A positive pregnancy test result and an ultrasound that does not reveal the location is known as a pregnancy of unknown location (PUL). [3] Occasionally, a normal intrauterine pregnancy does result. Depending on the clinical scenario, a clinician may choose to observe this patient with serial hCG levels and ultrasonography instead of intervening, or a clinician may need to intervene depending on the situation.


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