Twin pregnancies involve distinct biological processes that determine how twins are nourished and supported. The arrangements of the placenta and surrounding membranes vary, influencing the course and management of these unique pregnancies.
Understanding Twin Development
Twins develop through two primary biological pathways, leading to either fraternal or identical siblings. Fraternal, or dizygotic, twins result from the fertilization of two separate eggs by two distinct sperm cells. They share approximately 50% of their DNA, like any other siblings. They can be of the same sex or different sexes, reflecting independent genetic contributions.
In contrast, identical, or monozygotic, twins originate from a single fertilized egg that splits into two embryos early in development. They share virtually identical genetic material and are always of the same sex. The timing of this initial cell division significantly determines how their placentas and membranes will arrange.
Placental Arrangements in Twin Pregnancies
Fraternal twins typically implant individually. Each develops its own distinct placenta and amniotic sac. This arrangement is dichorionic-diamniotic, meaning two chorions (outer membranes) and two amnions (inner sacs).
While fraternal twins always start with two separate placentas, these organs can sometimes grow so close that they appear as a single, fused mass on ultrasound. This fusion occurs when implantation sites are adjacent, causing superficial merging. Despite this proximity, fused placentas maintain separate blood supplies, with no shared vessel connections. True placental sharing, with inter-twin vascular connections, is characteristic of certain identical twin pregnancies and carries different implications.
Identical twins can exhibit various placental configurations depending on when the single egg divides. If the split happens very early (within the first few days), identical twins can also be dichorionic-diamniotic, having their own separate placentas and amniotic sacs. If division occurs later (typically between four and eight days), identical twins share a single placenta but develop in separate amniotic sacs (monochorionic-diamniotic). In rare instances, if the split happens even later, identical twins may share both a single placenta and a single amniotic sac (monochorionic-monoamniotic), which is the least common and highest-risk configuration.
The Significance of Chorionicity and Amnionicity
Chorionicity and amnionicity describe the placental and membrane arrangements in twin pregnancies. Chorionicity refers to the number of chorionic membranes, indicating the number of placentas (separate or fused). Amnionicity refers to the number of amniotic sacs surrounding each fetus.
Determining these factors early in a twin pregnancy, through ultrasound, helps healthcare providers understand potential risks and tailor monitoring plans. Monochorionic pregnancies, where twins share a single placenta, are associated with a higher risk of specific complications.
One complication is Twin-to-Twin Transfusion Syndrome (TTTS), occurring with unequal blood vessel connections within the shared placenta. This leads to uneven blood flow, causing one twin to receive too much and the other too little. Other risks for monochorionic twins include selective fetal growth restriction and, for monoamniotic twins, umbilical cord entanglement. Since fraternal twins are almost always dichorionic-diamniotic, with their own placenta and amniotic sac, they do not face shared placental complications like TTTS.