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You’ve heard the stories: An unsuspecting couple goes to the hospital to have a baby and ends up leaving with two babies because — surprise! — there were actually twins in there. A whole other baby nobody knew about… or saw on any of the ultrasounds… or had any reason to believe was playing the world’s longest game of hide and seek.

Does this happen? Sure. That said, it’s nowhere near as common as many people think. Thanks to advances in modern imaging technology, it’s hard for twins to conceal themselves beyond a certain point in pregnancy.

So while you could be fooled in the first trimester (and maybe even into the second), by the time you get to the third trimester, it’s pretty darn difficult for an ultrasound to overlook an entire human being in your uterus.

Here’s what you need to know about “hidden twins,” from why it happens to when it’s nearly impossible for that little superspy to remain undetected any longer.

Yes! Ultrasounds aren’t perfect. Classic example: What looks like a penis turns out to be a vulva, and whoops, you’ve got a baby girl — not a baby boy. Because ultrasounds are sonographic images of what’s going on beneath the surface, you have to go into them leaving some room for error.

The bigger and more developed your baby is, the less likely it is they could be hiding. At the same time, the earlier you are in your pregnancy, the more possible it is an additional baby is reluctant to make their ultrasound debut.

What are some of the most common scenarios that can lead to a hidden twin?

  1. Your first ultrasound happens early in pregnancy. The earlier your ultrasound, the less accurate it is. We’re talking super small, itty-bitty humans here — and even when yolk sacs and fetal poles are developing, it’s not always possible to see them until they’re a smidge bigger. If you have a transvaginal ultrasound at 6 or 7 weeks, you might only see one baby when there’s really two.
  2. Your twins share an amniotic sac. Sometimes twins develop their own individual sacs, but when they share one, it increases the likelihood that one twin could be hiding during an early ultrasound. These twins are called monochorionic monoamniotic (or mo/mo twins, for short).
  3. Your hidden baby is super stealthy. In other words, your baby parked themselves behind their sibling, ducked into one of your uterus’ nooks and crannies, or slipped juuuuust out of view of the ultrasound during the exam.
  4. Lack of prenatal care. A single ultrasound isn’t really quite sufficient to roll with for the entire 9 months. If you’re not able to get regular ultrasounds during pregnancy, you could end up surprised at birth. (This is why a surprise twin delivery was much more common prior to ultrasound technology.)
  5. Human error. The people who interpret ultrasound technology are only human, after all. Even an experienced technician or healthcare provider could misinterpret that mishmash of light and dark shapes, mistaking a developing twin for something else. (Again, this is most likely with first-trimester ultrasounds.)

OK, so you can’t be 100 percent sure how many babies you’ve got in your uterus until you give birth (hence the urban legends).

But generally, you can be 99.99 percent sure there’s not a twin hiding somewhere after 20 weeks. This is usually the time when you have a super detailed anatomy scan to assess your baby’s growth and development. It’s highly unusual that a twin could still be discreetly chilling, unseen, in the background at this point. They’re just too big to be missed.

Before 20 weeks, though? All bets are off.

Are there other ways to know you’re having twins if an early ultrasound doesn’t detect multiple babies? Well, the symptoms of a twin pregnancy are pretty much the same as those of a singleton one, just heightened or more intense. But you can be on the lookout for the following:

  • severe nausea or morning sickness (hyperemesis gravidarum is more common in twin pregnancies)
  • excessive weight gain or fatigue
  • abnormally high levels of hCG, a.k.a. the pregnancy hormone
  • repeatedly measuring large for your gestational age at your prenatal exams

When it comes to twins, nothing is off the table! Technically, a twin can hide out in your uterus, but only for so long.

It’s not unheard of for a twin pregnancy to go undetected in early ultrasounds (say, around 10 weeks). But once you reach the midway point of your pregnancy and have your 20-week anatomy scan, you can be 99.99 percent confident about how many babies to expect at your delivery.