With the ready availability of ultrasound, fewer than 10% of twin gestations are undiagnosed before labor and delivery. Early diagnosis facilitates appropriate prenatal care.
A. Symptoms and Signs
All of the common annoyances of pregnancy are more troublesome in multiple pregnancy. The effects of multiple pregnancy on the patient include earlier and more severe pressure in the pelvis, nausea, backache, varicosities, constipation, hemorrhoids, abdominal distention, and difficulty in breathing. A “large pregnancy” may be indicative of twinning (distended uterus).
Fetal activity is greater and more persistent in twinning than in singleton pregnancy. The median weight of twins at birth is just over 2270 g in the United States. Male infants weigh slightly more than females.
Considering the possibility of multiple pregnancy is essential to early diagnosis. If one assumes that all pregnancies are multiple until proved otherwise, physical examination alone will identify most cases of twinning before the second trimester. Indeed, diagnosis of twinning is possible in over 75% of cases by physical examination. The following signs should alert the physician to the possibility or definite presence of multiple pregnancy:
(1) Uterus larger than expected (> 4 cm) for dates.
(2) Excessive maternal weight gain that is not explained by edema or obesity.
(3) Polyhydramnios, manifested by uterine size out of proportion to the calculated duration of gestation, is almost 10 times more common in multiple pregnancy.
(4) History of assisted reproduction.
(5) Elevated MSAFP values (see Laboratory Findings, below).
(6) Outline or ballottement of more than one fetus.
(7) Multiplicity of small parts.
(8) Simultaneous recording of different fetal heart rates, each asynchronous with the mother’s pulse and with each other and varying by at least 8 beats per minute. (The fetal heart rate may be accelerated by pressure or displacement.)
(9) Palpation of one or more fetuses in the fundus after delivery of one infant.
Some of the common complications in early pregnancy may also occur as a result of multiple gestation. For example, maternal bleeding in the first trimester can indicate threatened or spontaneous abortion; however, the dead fetus may be one of twins, as demonstrated by real-time ultrasonography (one anechoic or hypoechoic amniotic sac and one normal sac). In the second and third trimester, the demise of one fetus in a multiple gestation may trigger disseminated intravascular coagulation (“dead fetus syndrome”), just as a singleton intrauterine demise might. This generally becomes a problem only 3 weeks or more after fetal demise. Preeclampsia-eclampsia is a common complication of multiple pregnancy.
B. Laboratory Findings
The majority of multiple pregnancies are currently identified by using maternal serum alpha-fetoprotein (MSAFP) screening or ultrasound. Indeed, identification of multiple gestation is so important for the institution of appropriate care that many authorities recommend routine ultrasonic scanning at 18-20 weeks. First-trimester ultrasonography is even more helpful for determining chorionicity.
The hematocrit and hemoglobin values and the red cell count usually are considerably reduced, in direct relationship to the increased blood volume. Indeed, maternal hypochromic normocytic anemia occurs so frequently in multiple pregnancy that it has been suggested that all patients with the process be suspected of having a multiple gestation. Fetal demand for iron increases beyond the mother’s ability to assimilate iron in the second trimester.
Glucose tolerance tests demonstrate that both gestational diabetes mellitus and gestational hypoglycemia are much higher in multiple gestation compared with findings in singleton pregnancy. Glucose screening is the standard of care in multiple pregnancy.
C. Prenatal Diagnosis
The usual indications for prenatal diagnosis and counseling in a singleton pregnancy also apply to twin and higher-order gestations. Because the incidence of twin gestation increases with maternal age, women with multiple gestations are often candidates for prenatal genetic diagnosis. As the risk of aneuploidy is increased, some centers offer invasive testing to all patients carrying multiple gestations who will be over age 33 at delivery. Genetic counseling should make clear to the patient the need to obtain a sample from each fetus, the risk of a chromosomal abnormality, potential complications of the procedure, the possibility of discordant results, and the ethical and technical concerns when one fetus is found to be abnormal.
In twin pregnancies not accompanied by neural tube defects, the median MSAFP level will be 2.5 that of the median level for singleton pregnancies at 14-20 weeks’ gestation. The levels in triplets and quadruplets are 3 and 4 times as high, respectively. A value greater than 4.5 times the median is considered abnormal, and requires a targeted ultrasound and possible amniocentesis for the determination of amniotic fluid alpha-fetoprotein and acetylcholinesterase.
Both amniocentesis and chorionic villus sampling (CVS) can safely be performed in multiple gestations in experienced centers. Documentation of the location of the fetuses and the membrane separating the sacs is important in case there is discordance for aneuploidy. Selective termination of an aneuploid fetus can be performed via ultrasound-guided intracardiac injection of potassium chloride. The pregnancy can then continue carrying the normal twin only. Multifetal reduction may be performed to decrease the risk of serious perinatal morbidity and mortality associated with preterm delivery by reducing the number of fetuses from 3 or more to twins.
D. Ultrasound Findings
Ultrasonography is the preferred imaging modality for diagnosis of multiple gestation, and is potentially able to differentiate multiple gestation as early as 4 weeks (by intravaginal probe). Dichorionicity is suggested by fetuses of different genders, separate placentas, a thick (> 2 mm) dividing membrane, or a “twin peak sign” in which the membrane inserts into two fused placentas. In the absence of these findings, monochorionicity is likely.
Both twins present as vertex in almost 50% of cases. Twin A will be vertex and twin B a breech in slightly more than 33% of cases (
Fig 17-3). Both fetuses will be breech presentations in 10% of cases, and almost that many will be single (or double) transverse presentations. Approximately 70% of first twins present by the vertex. Breech presentation occurs in slightly more than 25%. Overall, nonvertex presentation occurs 10 times more often in multiple pregnancy than in singleton pregnancy.
Figure 17-3. Left: Both twins presenting by the vertex. Right: One vertex and one breech presentation. (Reproduced, with permission, from Benson RC: Handbook of Obstetrics & Gynecology, 8th ed. Lange, 1983.)
Revision date: July 3, 2011
Last revised: by David A. Scott, M.D.