What factors influence identical twin pregnancies following ART?

This study investigated factors affecting monozygotic twins (MZT) rates in patients undergoing fertility treatment.

They found that embryo stage at transfer and higher embryos numbers increased the risk of MZT.

Monozygotic is the term used to describe identical twins. Monozygotic twins (MZT) occur when two embryos form from a one fertilized egg. MZT are 2-12 times more common in pregnancies after assisted reproduction (ART). This includes in vitro fertilization or IVF.

MZT pregnancies carry a higher risk than single pregnancies. There is an increased risk of pregnancy loss, preterm delivery, and labor complications. Women with MZT pregnancies have a higher risk of gestational diabetes, hypertension, and preeclampsia.  It is unclear why MZT are more common after ART. Some studies suggest IVF procedures may affect the rate of MZT.

This study included 11,501 women who got pregnant by ART. Data on ART procedures were analyzed between MZT and single pregnancies. The authors looked at the methods used for IVF. The time spent culturing (growing) fertilized embryos before the transfer was compared. The quality of embryos was also compared.

The embryo stage at transfer was associated with MZT rates (462%).  The longer the embryo spent in culture before transfer to the womb, the higher the risk of MZT. The risk of MZT occurs in embryos transferred at stage 3.5 or greater. The number of good-quality embryos also increased MZT risk (128%). Good-quality embryos greater than 3.5 significantly increased MZT risk.

The authors concluded that the embryo stage at transfer and higher embryos numbers increased the risk of MZT.

This study was conducted in a Chinese population. The methods used may differ in other countries.  The results may also differ. Further investigation is needed. 

Please consult with your doctor if you have any concerns regarding MZT pregnancy.