Every 2-week ultrasounds outperform 4-week for dichorionic twins

Everyone loves twins. Double the baby, double the trouble, double the fun. However, twin pregnancy are not all fun and games. We have long known that they are at increased risk of complications, such as fetal growth restriction. There are different types of twin pregnancies, depending on the number of placentas and amniotic sacs around the babes. When the two babies share a placenta, it is known as a monochorionic twin pregnancy. These pregnancies are at increased risk of unique complications such as twin-twin transfusion syndrome and unequal placental sharing. For this reason, obstetricians do ultrasound scans of monochorionic twin pregnancies every two weeks starting at 16 weeks gestation.

What about twins that don’t share a placenta: dichorionic twin pregnancies? How often should we do ultrasounds for them? They are still at increased risk of pregnancy complications. A large research consortrium of 8 specialty obstetric hospitals in Ireland, led by Dr. Corcoran, completed a study published in the October 2015 Grey Journal to find out. They were very careful with their twin mommas. All 789 dichorionic twin pregnancies got an ultrasound every 2 weeks starting at 24 weeks gestation. The moms’ healthcare was managed according to the findings of these ultrasounds, and they had a low overall rate of perinatal complications or death.

In order to create comparison groups, the authors simply took the same group of moms and simulated what would have happened if they only got ultrasounds every 4 weeks. Essentially, they deleted half the ultrasound results! They could then compare the differences in detection rates of ultrasound findings of complications between 2-week and 4-week ultrasounds. They found a significantly decreased detection rate for the 4-week ultrasound plan as compared to the 2-week ultrasound plan. The 2-week plan detected 88% of fetal growth restriction, but the 4-week plan only detected 69%. The 2-week plan detected 82% of abnormal umbilical artery Dopplers (blood flow in the umbilical cord), while the 4-week plan detected 62%. Finally, the 2-week plan detected 32% of low amnionitic fluid and the 4-week plan detected 23% (a non-significant trend).

The authors took the study one step further: what if you do a combined approach? They also compared the 2-week and 4-week plans to performing an ultrasound every 4 weeks from 24 to 32 weeks gestation and then switching to every 2 weeks thereafter. This plan’s detection rates were not significantly different from the 2-week plan.

The authors conclude that obstetricians should start doing ultrasounds every 2 weeks instead of every 4 weeks for dichorionic twin pregnancies, but their research also supports doing ultrasound every 4 weeks from 24-32 weeks gestation and then every 2 weeks for the rest of the pregnancy. Future research is still needed to study if the starting dates of these plans are ideal, or if we should start all these ultrasounds earlier or later in pregnancy.