What does it mean to have two sacs at birth but only one baby?

Monochorionic diamniotic (MCDA) twins or triplet pregnancies occur when two, or sometimes three, babies share the same placenta (monochorionic) but have separate amniotic sacs (diamniotic). Monochorionic twins resulting from assisted conception seem to have roughly the same rate of complications as spontaneously conceived monochorionic twins. When the zygote splits early, by day three or four after conception, the twins will have completely separate placentas and amniotic sacs (dichorionic diamniotic or DCDA twins). MCDA twins are more likely to result in conjoined twins and monoamniotic twins than DCDA twins, but monozygotic twins occur 75% of the time in monochorionic pregnancies and 25% in dichorionic pregnancies.