Pathologic Factors Associated with Twinning

Although the blood volume is increased in multiple pregnancy, maternal anemia often develops because of greater demand for iron by the fetuses. However, prior anemia, poor diet, and malabsorption may precede or compound iron deficiency during multiple pregnancy. Respiratory tidal volume is increased, but the woman pregnant with twins often is "breathless" (possibly due to increased levels of progesterone).

Marked uterine distention and increased pressure on the adjacent viscera and pelvic vasculature are typical of multiple pregnancy. Lutein cysts and even ascites are the result of abnormally high levels of chorionic gonadotropin in occasional multiple pregnancies. Placenta previa develops more frequently because of the large size of the placenta or placentas.

The maternal cardiovascular, respiratory, gastrointestinal, renal, and musculoskeletal systems are especially subject to stress in multiple pregnancy, combined with greater maternal-fetal nutritional requirements. Multiple pregnancy is classified as high-risk because of the increased incidence of maternal anemia, urinary tract infection, preeclampsia-eclampsia, hemorrhage (before, during, and after delivery), and uterine inertia.

A. Placental and Cord

The placenta and membranes of monozygotic twins may vary considerably (Fig 17-1), depending on the time of initial division of the embryonic disk. Variations are noted below.

(1) Division prior to the morula stage and differentiation of the trophoblast (third day) results in separate or fused placentas, 2 chorions, and 2 amnions. (This process grossly resembles dizygotic twinning and accounts for almost one-third of monozygotic twinning.)

(2) Division after differentiation of the trophoblast but before the formation of the amnion (fourth to eighth days) yields a single placenta, a common chorion, and 2 amnions. (This accounts for about two-thirds of monozygotic twinning.)

(3) Division after differentiation of the amnion (8th-13th days) results in a single placenta, 1 (common) chorion, and 1 (common) amnion. This is rare.

(4) Division later than day 15 may result in incomplete twinning. Just prior to that time (day 13-15), division may result in conjoined twins.

At delivery, the membranous T-shaped septum or dividing membrane of the placenta between the twins must be inspected and sectioned for evidence of the probable type of twinning (Fig 17-2). Monozygotic twins most commonly have a transparent (< 2 mm) septum made up of 2 amniotic membranes only (no chorion and no decidua). Dizygotic twins almost always have an opaque (thick) septum made up of 2 chorions, 2 amnions, and intervening decidua.

A monochorionic placenta can be identified by stripping away the amnion or amnions to reveal a single chorion over a common placenta. In virtually every case of monochorionic placenta, vascular communications between the 2 parts of the placenta can be identified by careful dissection or injection. In contrast, dichorionic placentas (of dizygotic twinning) only rarely have an anastomosis between the fetal blood vessels.