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

Back in the old days, women didn't know if they were having twins until they actually gave birth, which probably made designing the nursery more stressful than it had to be. These days, thanks to ultrasounds, most women know early whether they'll be changing double diapers or not. Neverthless, friends and family love to double check well into the third trimester. ("Are you sure it's not twins?" "Yes, mom, I'm sure.") In pursuit of the final word, Romper reached out to doctors to answer this burning question once and for all: how long can twins go undetected? Is a surprise second baby really a thing?

According to Very Well Family, hidden twins were possible only a decade ago, but today, improved ultrasound technology means it's super unlikely that you're harboring a camera-shy second fetus. As Dr. Jamil Abdur-Rahman, OB-GYN and one-half of the Twin Doctors for TwinDoctorsTV, tells Romper in an interview, by week seven the actual baby or babies can be clearly seen, along with their heartbeats. And that beating heart (or hearts!) leaves very little room for doubt. "So, beyond that point, it would be tough to miss two babies on ultrasound,” he says.

Before week seven, though, detecting multiples is more dicey, because there's not nearly so much to be seen. "Prior to to the seventh week of pregnancy, the only signs of pregnancy may be a structure called a ‘gestational sac,'" explains Abdur-Rahman, 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 mono-amniotic twins are rare, occurring in less than 1 percent of all pregnancies, reported Stanford Medicine News Center, and the majority of babies will have their own amniotic sac from get-go.

According to Dr. Rebecca Levy-Gantt of Premier OBGYN Napa, Inc., 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 American Pregnancy Association reported, a vanishing twin is essentially a miscarriage involving only one baby. Sadly, it's pretty common, occurring in 21 to 30 percent of multiple pregnancies. 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.

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.

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.