Beginning in the first trimester, and temporarily associated with higher serum 3-hCG levels, women with a multifetal gestation oten have nausea and vomiting in excess of that with a singleton pregnancy. In women carrying more than one fetus, blood volume expansion is greater and averages 50 to 60 percent compared with 40 to 50 percent in those with a singleton (Pritchard, 1965). This augmented hypervolemia teleologically ofsets blood loss with vaginal delivery of twins, which is twice that with a single fetus. Although red cell mass also accrues, it does so proportionately less in twin pregnancies. Combined with greater iron and folate requirements, this predisposes to anemia.
Women carrying twins also have a typical pattern of arte rial blood pressure change. MacDonald-Wallis and coworkers (2012) analyzed serial blood pressures in more than 13,000 singleton and twin pregnancies. As early as 8 weeks' gestation, the diastolic blood pressure in women with twins was lower than that with singleton pregnancies but generally rose by a greater degree at term. An earlier study demonstrated that this rise was at least 15 mm Hg in 95 percent of women with twins compared with only 54 percent of women with a singleton (Campbell, 1986).
Hypervolemia along with decreased vascular resistance has an impressive efect on cardiac function. In one study of 119 women with a twin pregnancy, cardiac output rose another 20 percent above that in women with a singleton pregnancy (Kametas, 2003). Similarly, Kuleva and coworkers (201l) using serial echocardiography found a greater increase in cardiac output in 20 women with uncomplicated twin pregnancies. Both studies found the augmented cardiac output was predominantly due to greater stroke volume rather than higher heart rate. Vascular resistance was signiicantly lower in twin gestations throughout pregnancy compared with singleton ones. In a study of 30 uncomplicated twin pregnancies, this same group of investigators using echocardiography later identified progressive diastolic dysfunction from the irst to third trimester. The dysfunction subsequently normalized after delivery (Ghi, 2015).
Uterine growth in a multifetal gestation is substantively greater than in a singleton pregnancy. The uterus and its nonfetal contents may achieve a volume of 10 L or more and weigh in excess of 20 pounds. Especially with monozygotic twins, excessive amounts of amnionic luid may rapidly accumulate. In these circumstances, maternal abdominal viscera and lungs can be appreciably compressed and displaced by the expanding uterus. As a result, the size and weight of the large uterus may preclude more than a sedentary existence for these women.
Ifhydramnios develops, maternal renal function can become seriously impaired, most likely as the consequence of obstructive uropathy (Quigley, 1977). With severe hydramnios, therapeutic amniocentesis may provide relief for the mother, may improve obstructive uropathy, and possibly may lower the preterm delivery risk that follows preterm labor or prematurely ruptured membranes. Unfortunately, hydramnios is often characterized by acute onset remote from term and by rapid reaccumulation despite amniocentesis.
Miscarriage is more likely with multifetal gestation. In one 16-year study, the spontaneous abortion rate per live birth in singleton pregnancies was 0.9 percent compared with 7.3 percent in multifetal ones (Joo, 2012). Also, twins achieved through ART are at greater risk for abortion compared with those conceived spontaneously (Szymusik, 2012).