Ultrasound evaluation of early pregnancy
Use of ultrasound (U/S) in obstetrics and gynecology dates back to 1958 when Donald et al. published their experience on identification of abdominal masses using U/S in the Lancet.10 Since then U/S evaluation has become nearly universal in developed countries as part of routine prenatal care and is used to diagnose pregnancy, establish its location and viability, and assess fetal anatomy and growth and placental position. Ultrasound has become a preferred modality for pregnancy viability assessment since 1987 when the transvaginal approach for this purpose became widely available.11,12 Since then criteria for a non-viable pregnancy have been suggested.
A pregnancy is termed viable if it has the potential to result in a live-born neonate.2,12 The two major criteria that must be met for a pregnancy to be viable are a normally location within the uterus and a potentially viable fetus.
Timely identification of a pregnancy’s normal location within the uterus is crucial. The diagnosis of intrauterine pregnancy is most commonly made with U/S, based on identification of an intrauterine gestational sac located toward the fundus with at least yolk sac or fetal pole with or without a heartbeat (Figure 1).
Findings of gestational sac but without yolk sac or fetal pole traditionally were not considered exclusive of abnormally located pregnancy due to a possibility of the visualized sac being a pseudosac in association with an ectopic pregnancy. This paradigm and thus the term pseudosac itself, however, have recently been challenged due to the predominance of occasions when a round fluid collection in the uterus in conjunction with a positive pregnancy test were indicative of an intrauterine pregnancy.13,14
Criteria for viability can vary at different gestational ages. In a viable singleton pregnancy, correlation between pregnancy dating, hCG level, and U/S findings has been described, though there is some variability in the milestones. The embryo is expected to grow by at least 0.2 mm/day, and the gestational sac at least 1 mm/day.15
In many instances a single point hCG measurement is not diagnostic of either the viability of pregnancy, or its location; rather hCG trend is much more helpful in distinguishing those. With a normal intrauterine pregnancy hCG is expected to rise by at least 53% over a 48-hour period, and a deviation from that trend may indicate a nonviable intrauterine or an abnormally located pregnancy. Importantly, observation of a normal rise does not necessarily exclude the possibility of an abnormal intrauterine or abnormally located pregnancy.18 Once a viable IUP is confirmed, hCG does not need to be trended further. To describe the correlation between the hCG level and U/S findings, the term “discriminatory zone” (Table) emerged and is used to determine the hCG value above which an intrauterine gestational sac is consistently seen on U/S in normal pregnancies.13 The discriminatory zone is institution-specific and usually ranges between 1500 and 3500 mIU/mL. However, care should be taken to personalize the use of discriminatory zone because there are certain clinical scenarios that result in deviation of the hCG curve and U/S findings from expected pattern. An example is early multiple-order gestations, which may not necessarily have the same association between the hCG level and U/S findings and higher discriminatory hCG level (~3000 mIU/mL).
None of the authors report a conflict of interest to report with respect to the content of this article.
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