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What Are Hidden Twins? An OB-GYN Explains

There’s no way to describe the overwhelming feeling you get when you first see your baby during an ultrasound. At first, your baby looks like a tiny kidney bean, but by that second trimester ultrasound, you get to finally see the form of a baby, which makes the rest of your pregnancy even more exciting. But if multiples run in your family, or you feel like you are carrying more than one baby, you may wonder if there is a baby hidden in there somewhere. But can that happen? What are hidden twins?

In an interview with Romper, OB-GYN and medical travel blogger from TwinDoctorsTV, Dr. Idries Abdur-Rahman, says that hidden twins are the instance when a twin pregnancy goes undiscovered. He explains that while this oversight is more common in women who receive little to no prenatal care, there have still been some documented hidden twin pregnancies in women that had complete prenatal care.

“A hidden twin on ultrasound is most likely to occur with early ultrasounds during the first trimester,” says Abdur-Rahman, “because the smaller the embryo, the more difficult it is to see the baby and the baby's heartbeat.” He says that usually, a hidden twin on an early ultrasound will be seen later on the second trimester ultrasound, like the 5-month anatomy ultrasound. Even though it is a rare phenomenon, he notes that there have been documented cases of a hidden twin in pregnant mothers who actually had multiple ultrasounds.

When your doctor looks at your ultrasound, he is essentially looking at an image of your womb, explained VeryWell Family, but in the early months of pregnancy, while rare, it’s possible to miss seeing a twin. It has to do more with positioning, the article noted, and in the rare case that one baby is positioned directly behind the other, it may not be visible on a two-dimensional ultrasound. This is more likely when twins are mono-chorionic (in one single chorionic sac), which causes the two babies to be positioned so close to each other that only one is detected on the ultrasound.

If you have a strong feeling that you are carrying twins, but see only one baby on your ultrasound, you might be curious to know if there are other ways to tell how many babies you are carrying. Abdur-Rahman says that the only definitive way to diagnose a twin pregnancy is by ultrasound, even though it’s not always perfectly accurate. He explains that blood tests for the pregnancy hormone, beta hCG, can give you an indication early on that a mom might be carrying twins, because in multiple pregnancies, the beta hCG level is significantly higher than in single pregnancies. “Having said that, there are other pregnancy related conditions that can cause the beta hCG level to be higher than normal,” adds Abdur-Rahman, “so a higher level may be a clue to a twin pregnancy, but the only foolproof way to diagnose a twin pregnancy is to have an ultrasound done.”

So while hidden twins are uncommon, there have been cases where women end up delivering two babies by surprise. In fact, on Twinversity, there are dozens of accounts where women tell their stories of finding out about their hidden twin pregnancy. If you feel like your baby is not alone in there, talk to your doctor about it. You could be asked to have another ultrasound, or you might even consider getting a 3D ultrasound. Either way, discussing your concerns with your doctor is the best way to get to the bottom of your baby mystery.

Check out Romper's new video series, Bearing The Motherload, where disagreeing parents from different sides of an issue sit down with a mediator and talk about how to support (and not judge) each other’s parenting perspectives. New episodes air Mondays on Facebook.