Acardiac twin pregnancies part VI: Why does acardiac twinning occur only in the first trimester?

Background Clinical observation suggests that acardiac twinning occurs only in the first trimester. In part, this contradicts our previous analysis (part IV) of Benirschke's concept that unequal embryonic splitting causes unequal embryo/fetal blood volumes and pressures. Our aim is to explain why acardiac onset is restricted to the first trimester. Methods We applied the vascular resistance scheme of two fetuses connected by arterio‐arterial (AA) and veno‐venous (VV) anastomoses, the small VV resistance approximated as zero. The smaller twin has volume fraction α