Twin pregnancies are generally managed with a higher level of caution than singleton pregnancies. Dichorionic Diamniotic (Di/Di) twins represent the lowest-risk category of all twin types. This classification means each baby has its own separate support system within the uterus, allowing the pregnancy to progress further than other twin gestations. Delivery timing balances the benefits of continued development inside the womb with the increasing risks of staying too long.
Understanding Dichorionic Diamniotic Twins
Dichorionic Diamniotic twins are characterized by having two distinct placentas and two separate amniotic sacs. The term “Dichorionic” refers to the two outer membranes (chorions) that form the placentas, while “Diamniotic” indicates the two inner sacs (amnions) that hold the amniotic fluid and the babies. This anatomical separation provides each twin with an independent supply of nutrients and oxygen, significantly reducing shared risks.
This setup is characteristic of all fraternal (dizygotic) twins, who result from two separate fertilized eggs. It can also occur in identical (monozygotic) twins if the fertilized egg splits early enough. The presence of two separate placentas distinguishes Di/Di pregnancies from higher-risk types, such such as monochorionic twins. Monochorionic pregnancies share a single placenta, which can lead to complications like Twin-to-Twin Transfusion Syndrome (TTTS) due to shared blood vessels.
Since Di/Di twins do not share a placenta, they avoid complications associated with uneven blood flow that can affect monochorionic pregnancies. This individual resource management is the main reason why Di/Di pregnancies can be carried further into gestation. Despite this lower risk, they are still multiple gestations and carry a higher risk of complications like preterm birth and preeclampsia compared to a single baby.
Consensus on Delivery Timing
Delivery timing for uncomplicated Di/Di twins is guided by medical consensus to optimize outcomes. Current guidelines from organizations like the American College of Obstetricians and Gynecologists (ACOG) recommend a planned delivery between 38 weeks, 0 days and 38 weeks, 6 days of gestation. This window is chosen because the benefits of continued fetal maturation begin to be outweighed by the risks of remaining in the womb.
A full-term pregnancy for a single baby is 40 weeks, but a twin pregnancy reaching 38 weeks is generally considered a successful outcome. The recommendation is to schedule an induction or cesarean delivery within this week to prevent adverse events. Delivery before 38 weeks may be considered to avoid potential complications, but the goal remains to reach this optimal period.
The purpose of this timing is to prevent the increased risks that occur near the end of a multiple gestation. Reaching the 38th week allows the twins to gain sufficient weight and lung maturity within the safe delivery window. This planned approach ensures the medical team is prepared for the delivery, which is safer than an emergency scenario.
Medical Justification for Intervention
Medical providers intervene with a scheduled delivery because fetal risk increases if the Di/Di pregnancy continues beyond 38 weeks. Studies indicate that the risk of stillbirth begins to rise as the twins approach and pass 39 weeks of gestation. The relative safety of continuing the pregnancy drops sharply, making delivery the safer option.
This heightened risk is largely attributed to the aging of the placenta, which is supporting two fetuses instead of one. As the placenta ages, its ability to efficiently deliver oxygen and nutrients, a condition known as placental insufficiency, can decrease. This decline in function can lead to complications such as restricted fetal growth and sudden fetal compromise.
Delivery may be scheduled earlier than the standard window if complications arise during the pregnancy. Conditions such as preeclampsia, which is more common in twin gestations, or significant growth discordance may necessitate delivery between 34 and 37 weeks. The medical team determines that the immediate risks of the complication outweigh the risks associated with a late-preterm delivery.