How Long Can Twins Go Undetected? Here's What You Should Know About Mystery Multiples

Ultrasounds haven’t always been around. Even now that they’re commonplace, they can tell us a lot about pregnancy and what’s going on inside the womb, but they are not flawless. Ask anyone who has decorated an entire nursery based on the expected biological sex of their baby only to discover after birth that they need to find all the receipts and tags. Imagine being pregnant with twins that are undetected. Now that’s a surprise. Luckily, it’s rare.

Nevertheless, friends and family love to double-check well into the third trimester. "Are you sure it's not twins?" Early in pregnancy, though, detecting multiples is a little bit trickier because there's not nearly so much to be seen. "Prior to the seventh week of pregnancy, the only signs of pregnancy may be a structure called a gestational sac," explains OB-GYN Dr. Jamil Abdur-Rahman, M.D., and occasionally, identical twins may share that protective bag of amniotic fluid. So theoretically, a doctor verifying an early pregnancy could miss the presence of a second baby before week six or seven. But twins sharing a sac, called mono-amniotic twins, are rare, occurring in about less than 1% of all pregnancies, according to Verywell Family. The majority of babies will have their own amniotic sac from the get-go.

But Dr. Rebecca Levy-Gantt, M.D., of Premier OBGYN Napa, Inc. says there are other reasons a doctor might miss a second baby before the middle of the first trimester — if there's a problem with one of the early embryos, for instance. Detection can be difficult if one sac is larger than the other, she tells Romper in an email interview, or if one of the babies is non-viable and not developing properly. Even so, she puts the absolute cut-off for missing a second twin at 10 weeks. After that, it's "very unlikely," she says.

"There is a syndrome called a vanishing twin where one of the twins stops developing after it was already identified as a second fetus," says Levy-Gantt. "In this case, we continue to see one baby develop pretty normally, but the second one shrinks and eventually disintegrates." As Healthline reported, a vanishing twin is essentially a miscarriage involving only one baby. Before the ultrasound existed, most parents would never have known about the lost twin. The very fact that parents today do know is evidence of how much information these early ultrasounds provide. Advances in ultrasound technology have also shown us that vanishing twin syndrome might be more common than we thought, according to Healthline.

What usually happens when a twin is hidden, according to Verywell Family, is that early ultrasounds might visualize only one embryo. A later ultrasound might reveal there actually is another baby in there, but positioning in the early ultrasound made them hard to see. Fortunately, the accuracy of ultrasound has increased in many ways. A 2018 study in the journal Ultrasound found that ultrasounds can now estimate the weight of baby in utero with more accuracy, but notes that there's still a lack of consistency. Like any technology, it highly depends on the skill of the person operating it.

With that said, it’s just not that likely you’ve been told you’re carrying one baby only to find you’re expecting double trouble. Pregnancy is full of surprises, but thanks to the magic of ultrasounds, a surprise twin after week seven — or week 10, at the very latest — just isn't realistic. Obstetric technology is simply too advanced to leave parents in the dark for long. At six weeks, an early ultrasound can detect an ectopic pregnancy, and give you a due date to last the whole nine months. You'll probably even hear the thump of your baby's heart. So the next time someone stops you on the street (or at a family brunch) to ask if you sure you're not having twins, you can say with certainty: one heartbeat, one baby.

Experts:

Dr. Jamil Abdur-Rahman, OB-GYN

Dr. Rebecca Levy-Gantt, Premier OBGYN Napa, Inc