Different Size Twins and Due Dates During Pregnancy

I got a question from a mom who is about 29 weeks pregnant with twins. She says that at the last ultrasound, one of her twins was measuring 30 weeks and the other twin was measuring 29 weeks. She asks if that’s a reason for concern. Is it normal for twins to be different sizes?

Is it cause for concern? It depends. During our twin pregnancy, our girls did measure differently throughout the twin pregnancy. One was consistently bigger than the other, just like you’re describing. The ultrasound technicians and the doctor used size to determine due dates, but you can always view those as just estimates. Like in your example, one was 29 weeks and one was 30 weeks. Those estimates are based off the size of the babies.

Estimates and Averages

Keep in mind that twins do come really early, typically around 36 weeks, compared to 40 weeks for a singleton pregnancy. I would go off the earlier due date if you’re trying to plan out your babies’ arrival. Talk to your doctor about your concerns about different size twins.

Potential Risk

There is something called twin-to-twin transfusion syndrome, or TTTS. That is a disease of the placenta that affects individual identical twins during the pregnancy. Essentially, blood can be transfused disproportionately from one twin, who we’ll call the donor, to the other twin, who is the recipient. This is a serious condition that would result in drastically different size babies. Here’s a more in-depth article on twin-to-twin transfusion syndrome, what it means, and the odds that it could happen.

Twins with Different Due Dates

Can twins have different due dates?

It is natural for twins to be different weights and thus have different due dates.

However, once it is time for delivery, both twins will be born. The doctor doesn’t leave one twin in utero just because the due date hasn’t arrived yet.

Different sizes lead to different due date estimates. These size differences will likely hold true at birth too.

In fact, when our girls arrived, they were both different sizes. One weighed more than the other. Odds are that your twins, even though they may be identical, will be different sizes when they arrive.

If you need other help staying healthy and keeping the babies healthy through the twin pregnancy, I recommend that you check out the second chapter of my book, the “Dad’s Guide to Twins.”

This topic was originally addressed on the Dad’s Guide to Twins Podcast Episode 81: Dogs and Twins, Psychology of Raising Twins, Different Due Dates.

Picture by weatherking.