First-Trimester Complications

Placental Hematomas and Subchorionic Hemorrhage

The embryonic placenta, or frondosum, may become detached, resulting in the formation of a hematoma, which typically causes vaginal bleeding. Most of these hemorrhages are contiguous with a placental edge. Although no risk factors have been associated with first-trimester placental separation, it has been reported to have a 50% or greater fetal loss rate. Although the prognosis seems to depend on the size of the hematoma, no specific volumes have been correlated in the first trimester with fetal outcomes. That said, improved outcomes do seem to be consistent with smaller hematomas.

Sonographically, placental hematomas may be difficult to distinguish from subchorionic hemorrhages. Patients who present with placental hematomas generally do not have symptoms, bleeding, or spotting because the bleed is within the chorionic sac and has no communication with the endometrium.

Subchorionic Hemorrhage

The most common occurrence of bleeding in the first trimester is from subchorionic hemorrhage. These low-pressure bleeds result from the process of implantation of the fertilized ovum into the endometrial cavity and myometrial wall. The hemorrhage is found between the myometrium and the margins of the gestational sac and may or may not be associated with the placenta. This finding can help to distinguish a subchorionic hemorrhage from abruptio placentae, which generally occurs in the second trimester and may present as a lucency posterior to the placenta. Clinical findings may include bleeding, spotting, or uterine cramping. If the hemorrhage becomes large enough, this can lead to spontaneous pregnancy loss (SPL).

Sonographic Findings

The appearance of bleeding varies with the stage of its organization. An early bleed may appear slightly echogenic as the red blood cells actively fill the area of hemorrhage. With time, the hemorrhage becomes more anechoic and may be seen between the uterine wall and the fetal membrane ( Fig. 50.1 ). Color flow Doppler will demonstrate the avascular nature of the hemorrhage. Patients may present with active vaginal bleeding, and the subchorionic bleed is easily seen by ultrasound adjacent to the gestational sac. Other patients may have no bleeding yet have a subchorionic lucency that can be seen with imaging. Patients may be symptomatic with a large subchorionic bleed, or asymptomatic with a small subchorionic bleed, perhaps only seen with transvaginal imaging.

Absent Intrauterine Sac

Sonography is routinely used to evaluate for the presence or absence of an intrauterine gestational sac. For example, if the patient presents with a positive pregnancy test with a normal sonographic appearance of the uterus but the endometrial complex shows no sign of a gestational sac, the differential diagnosis would include a very early intrauterine pregnancy, a nondeveloping pregnancy, or possible ectopic pregnancy.

Characteristics for the sonographic diagnosis of an absent intrauterine sac include an empty uterus with no evidence of an endometrial fluid collection (early gestational sac), absence of adnexal masses or free fluid, and positive beta-hCG levels. Clinical findings may be characterized by bleeding and cramping. Correlation between the serum beta-hCG level and uterine findings can be used to confirm whether the sonographic indications of a first-trimester pregnancy have been met. Recall that the gestational sac is identified sonographically at 4 to 5 weeks’ gestation. The sac grows approximately 1 mm per day in the first trimester. The yolk sac should be visualized transvaginally when the gestational sac reaches 8 mm in size, and the embryo must be visualized when the MSD measures 25 mm. The normal embryo grows at a rate of 1 mm/day. Cardiac activity is visible by 5.5 to 6.5 weeks ( Table 50.1 ). Failure to observe these developmental markers suggests a pregnancy of “uncertain viability” (PUV). Applying the SRU criteria, pregnancy failure can be definitively stated when the embryo is 7 mm or greater without a heartbeat or the MSD is 25 mm but no embryo is visible.

In the case of pregnancy loss ( complete abortion ) , serial hCG levels demonstrate successive decline. Caution should be taken when a positive pregnancy test and an empty uterus are seen, given the possibility that an early normal intrauterine pregnancy between 3 and 5 weeks’ gestation may be present. Consequently, serial hCG levels should always be obtained and followed for appropriate rise or decline.

If the endometrium is abnormally thick or irregularly echogenic, the differential diagnosis includes intrauterine blood, retained products of conception after an incomplete spontaneous loss, a decidual reaction associated with an ectopic pregnancy, or decidual changes resulting from an early but not yet visible intrauterine pregnancy. Incomplete spontaneous abortion may show several sonographic findings, ranging from an intact gestational sac with a nonviable embryo to a collapsed gestational sac that is grossly misshapen ( Fig. 50.2 ). Women who are clinically undergoing a spontaneous abortion or who have had an elective termination often require follow-up sonography to determine whether retained products of conception are present. Sonographic signs of retained products may be subtle; a thickened endometrium greater than 8 mm and increased vascularization of the endometrial complex with color Doppler are strongly predictive. The presence of visible embryonic parts, a gestational sac, or an embryonic disk is obvious evidence of retained products of conception ( Fig. 50.3 ). It sometimes can be difficult to distinguish retained products of conception from blood clots. Quantitative hCG levels, which do not decline normally, a thickened endometrium, and increased vascular flow will provide discriminating evidence for retained products. Patients most often present with persistent bleeding after SPL, dilation and curettage, or being prescribed misoprostol for uterine evacuation.

Gestational Sac Without an Embryo or Yolk Sac

A gestational sac without an embryo or yolk sac on sonography may represent one of three conditions: (1) a normal early intrauterine pregnancy of less than 5 weeks, (2) an abnormal intrauterine pregnancy, or (3) a pseudogestational sac in a patient with an ectopic pregnancy.

Criteria for Abnormal Sac

The gestational sac should be imaged consistently by both transabdominal and transvaginal sonography when its mean diameter is 5 mm, which corresponds to a gestational age of 4 to 5 weeks. Transvaginal sonography may demonstrate the sac as early as 4 weeks, when it measures 2 to 3 mm. At this early size the “sac” can be described only as an intrauterine fluid collection. This cannot be definitively called a gestational sac until there is further growth and a yolk sac can be identified. Measuring the gestational sac at this early stage provides a baseline for monitoring appropriate interval growth. Caution should be used in evaluating these early stages of pregnancy to allow for the possibility that a patient may have inaccurate dates for their last menstrual period. Sequential scanning can document appropriate interval growth of 1 mm per day. Lack of appropriate growth indicates an abnormally growing gestational sac or a pregnancy of unknown viability.

Regarding gestational sac values, a gestational sac may be seen as early as 4.3 weeks, the threshold level , and must be seen by 5.2 weeks, the discriminatory level . Given the new pregnancy failure criteria, absence of a gestational sac at this stage would also be termed PUV and follow-up ultrasound would be ordered.

Transvaginal sonography is the ideal method to examine the early gestational sac. When the sac measures 8 mm or greater, a definitive yolk sac should be demonstrated. The yolk sac can be expected to grow 0.1 mm per 1 mm of MSD growth, up to 15 mm. When the gestational sac measures 25 mm, an embryo with cardiac activity must be seen. Follow-up examination in 7 to 10 days is recommended if the findings are indeterminate.

Anembryonic Pregnancy (Blighted Ovum)

By definition, anembryonic pregnancy , or blighted ovum , is a gestational sac in which the embryo fails to develop or stops developing at such an early stage that it is imperceptible by ultrasound (see Table 50.1 ). The trophoblastic tissue may continue to proliferate despite the failed embryonic growth, the gestational sac will continue to grow, and hCG levels may continue to rise, although not at the expected rate. The typical sonographic appearance of anembryonic pregnancy is a large, empty gestational sac that does not demonstrate a yolk sac, an amnion, or an embryo ( Fig. 50.4 ). The MSD increases by 1.13 mm/day in a normal gestation, but the growth rate of an abnormal sac is only 0.70 mm/day. Therefore abnormal sac growth can be diagnosed when the MSD fails to increase by 0.6 mm/day. In subsequent repeat studies, the sonographer evaluates growth size of the sac, presence of the yolk sac, development of an embryo, and presence or absence of cardiac activity. Box 50.1 outlines several sonographic intrauterine findings associated with abnormal pregnancies. The SRU criteria for failed pregnancy in the presence of an empty sac is an MSD of 25 mm without an embryo.