Diagnosing twins by palpation of fetal parts before the third trimester is diicult. Even late in pregnancy, this may be challenging, especially if one twin overlies the other, if the woman is obese, or if there is hydramnios. Palpating two fetal heads, often in diferent uterine quadrants, strongly supports a twin diagnosis. Late in the first trimester, two fetal heartbeats may be diferentiated with Doppler ultrasonic equipment if their rates are clearly distinct from each other and from that of the mother.
Overall, however, using clinical criteria alone to diagnose multifetal gestations is unreliable. For example, in the Routine Antenatal Diagnostic Imaging with Ultrasound (RADIUS) trial, for 37 percent of women who did not have a screening ultrasound examination, their twin pregnancies were not diagnosed until 26 weeks' gestation. And, in 13 percent of unscanned women, their multifetal gestations were only diagnosed during their admission for delivery (American College of Obstetricians and Gynecologists, 2016; LeFevre, 1993).
Sonographic examination should detect practically all sets of twins. And, given the increased frequency of sonographic examinations during the first trimester, early detection of a twin pregnancy is common. Sonography can also be used to determine fetal number, estimated gestational age, chorionicity, and amnionicity. With careful examination, separate gestational sacs, if present, can be identiied early in twin pregnancy (Fig. 45-7). Subsequently, each fetal head should be seen in two perpendicular planes so as not to mistake a cross section of the fetal trunk for a second fetal head. Ideally, two fetal heads or two abdomens should be seen in the same image plane to avoid scanning the same fetus twice and interpreting it as twins.
Higher-order multifetal gestations are more challenging to evaluate. Even in the irst trimester, it can be diicult to identiY the actual number of fetuses and their position. This determination is especially important if pregnancy reduction or selective termination is considered (p. 891).
Abdominal radiography can be used if fetal number in a higherorder multifetal gestation is uncertain. However, radiographs generally have limited utility and may lead to an incorrect diagnosis if fetuses move during the exposure or if exposure time is inadequate. Additionally, fetal skeletons before 18 weeks' gestation are insuiciently radiopaque and may be poorly seen.
Although not typically used to diagnose multifetal pregnancy, magnetic resonance (MR) imaging may help delineate complications in monochorionic twins (Hu, 2006). In one review of 17 complicated twin gestations evaluated by both sonographic and MR imaging, the latter provided a more detailed assessment of twin pathology (Bekiesinska-Figatowska, 2013). This was particularly helpful in cases of conjoined twins.
No biochemical test reliably identifies multifetal gestations.
Serum and urine levels of 3-hCG and maternal serum levels of alpha-fetoprotein (MSAFP) are generally higher with twins compared with those in singletons. However, levels may vary considerably and overlap with those of singletons.
he various physiological burdens of pregnancy and the likelihood of serious maternal complications are typically greater with multifetal gestations than with a singleton pregnancy. his is considered, especially when counseling a woman whose health is compromised and in whom a multifetal gestation is recognized early. Similar consideration is given to the woman who is not pregnant but is considering infertility treatment.