The location of the pregnancy should be determined and documented. A definitive diagnosis of intrauterine pregnancy can be made when an intrauterine gestational sac containing a yolk sac or embryo/fetus with cardiac activity is visualized. Follow-up sonography and/or serial determination of maternal serum human chorionic gonadotropin levels are appropriate in pregnancies of undetermined location (see also Chap. 10) [1].

Although the visualization of multiple sacs early in the first trimester is suspicious, the diagnosis of a multiple pregnancy requires visualization of multiple embryos/fetuses. The first trimester is the optimum time to determine chorionicity and amnionicity, which are critical for management of multi-fetal pregnancies. The presence of separate sacs, the thickness of the intervening membrane, and the shape of its junction with the placenta should be assessed [2] (Fig. 6.1). Early in the first trimester, an intervening amnion may not be visible in diamniotic–monochorionic twins [2]. Amnionicity and chorionicity should be stated in the ultrasound report.

Although the term “viability” implies the ability to live independently outside of the uterus, from an ultrasound perspective, the term is used to describe the presence of an embryo with cardiac activity at the time of the examination [5]. An alternative definition, proposed by the SRU, is a pregnancy that can potentially result in a liveborn baby. Updated diagnostic criteria for pregnancy viability have recently been published by the SRU due to occurrences of women diagnosed with miscarriage or ectopic pregnancy that resulted in damaging interventions to potentially normal pregnancies [10]. Findings diagnostic of pregnancy failure using transvaginal ultrasound include a crown-rump length (CRL) of ≥7 mm and no heartbeat, mean sac diameter (MSD) of ≥25 mm and no embryo, absence of an embryo with a heartbeat ≥2 weeks after a scan that showed a gestational sac without a yolk sac, and absence of an embryo with a heartbeat ≥11 days after a scan that showed a gestational sac with a yolk sac. Findings suspicious for, but not diagnostic of, pregnancy failure using transvaginal ultrasound include a CRL of <7 mm and no heartbeat, MSD of 16–24 mm and no embryo, absence of an embryo with a heartbeat 7–13 days after a scan that showed a gestational sac without a yolk sac, absence of an embryo with a heartbeat 7–10 days after a scan that showed a gestational sac with a yolk sac, absence of an embryo ≥6weeks after LMP, empty amnion (amnion seen adjacent to yolk sac, with no visible embryo), enlarged yolk sac (>7 mm), and a small gestational sac in relation to the size of the embryo (<5 mm difference between MSD and CRL). This group has also proposed that specific terminology be used, as summarized in Table 6.2.

The MSD and CRL should routinely be measured and reported. The MSD is the average of three orthogonal measurements of the fluid filled space within the gestational sac (Fig. 6.2). The CRL is the maximum length of the entire embryo or fetus measured as a straight line in a true midsagittal plane (Fig. 6.3). Later in the first trimester, care should be taken to ensure that the fetus is not flexed by looking for the presence of fluid between the fetal chin and chest (Fig. 6.4). The ISUOG guidelines state that the biparietal diameter (BPD) and head circumference (HC) may also be measured after 10 weeks of gestation [5].