Yolk sac number, size and morphologic features in monochorionic monoamniotic twin pregnancy

To determine the relation between three characteristics of the yolk sac (number, size and morphologic features) and outcome in monochorionic monoamniotic twin pregnancy.

The authors reviewed data for the four sets of monochorionic monoamniotic twins detected by first-trimester ultrasonography between January 1990 and June 1994 at their institution. Data analysed included yolk sac number, size and morphologic features, as well as the outcome of the pregnancy.

In all four sets of twins, only one yolk sac was identified. In one case the yolk sac was irregular in contour, and in two it was abnormally large. Two of the four mothers delivered healthy twins at 34 weeks gestational age, one had conjoined twins ( and underwent elective termination of the pregnancy), and one had a twin ectopic pregnancy (and underwent salpingectomy).

A single yolk sac in cases of monochorionic monoamniotic twins may be a normal finding.