Vanishing Twin Syndrome

Reviewed by Dr. Elisa Ross in August 2010

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The rise in early-term ultrasounds has brought the Vanishing Twin Syndrome to light, revealing that a surprising number of twin pregnancies result in the birth of only one baby.

"We won the conception lottery!"

That's what I wrote in my online journal when I found out that my husband and I were expecting twins. After months of hormone injections, blood tests, ultrasounds, pills, ovulation test kits and egg extraction procedures, plus a heartbreaking miscarriage, we had finally hit the baby jackpot with the help of a fertility specialist and in vitro fertilization. The ultrasound image at my eight-week check-up showed two shrimp-like embryos and two little heartbeats flashing on the screen. My gynecologist gave me the phone number for a twins support group and told me to get ready for a wild ride!

Imagine my shock and horror when the next ultrasound, a mere two weeks later, showed one fat, wriggly embryo and one tiny bean-shaped one with no heartbeat. We were devastated. Grief stricken. How could this have happened? What went wrong? And what was going to happen to the surviving fetus?

The doctor said I was experiencing Vanishing Twin Syndrome, a common occurrence in the early stages of a multiple pregnancy. I might feel some cramping and possibly a tiny bit of bleeding, but the healthy baby would most likely survive and the "little one," as we called it, would be reabsorbed by my body over time and simply…vanish.

He was right. Although we were emotionally devastated by the loss of our tiny unborn baby, there was no discomfort or bleeding. My 20-week ultrasound revealed a healthy fetus and an empty amniotic sac, but no sign of the little twin. Our daughter was delivered healthy and on time, but we still think about what it would have been like to raise twins. We still sometimes miss the baby that came and went like a little ghost.

Act of Nature

As strange as it sounds, vanishing twins are pretty common. Before the advent of early ultrasounds, women didn't realize they were carrying multiple babies until the heartbeats could be heard at 12 weeks or until their regular five-month ultrasound. If two embryos had been conceived and one lost during the first trimester, a mother would never have known it.

Experts currently estimate that one-eighth of pregnancies begin as twins. But of course, the percentage of pregnancies that make it to term is smaller. Fertility specialist Dr. Carolyn Givens of the Pacific Fertility Center estimates that between 15 and 20 percent of all twin pregnancies will miscarry one fetus. "That's what happens what happens with a vanishing twin," she says, "You have an early miscarriage of the twin." Those estimates are supported by a 1986 sonogram study of 1,000 pregnancies. Exactly 21.9 percent of the women carrying twins in the first trimester experienced a "vanishing twin" event. Due to the increased use of fertility drugs and assisted fertility techniques like in vitro fertilization, there's been a spike in multiple pregnancies and an accompanying rise in early gestation ultrasounds, so what used to be an almost unheard of phenomenon is rapidly becoming a hot topic among mothers.

What Happens?

When our twin disappeared, I found myself wondering if I'd done something wrong to cause the embryo to die. Did I sleep in the wrong side? Inhale someone's second-hand smoke? Lug my heavy one-year-old daughter around too much? The fact is that most of the time a twin vanishes during the first trimester for the same reason a single baby miscarries: There's a fatal genetic problem with the embryo and it couldn't continue developing into a full-fledged fetus. Dr. Givens explains, "This usually happens within the first eight weeks because that's when the organs form, and if there's a problem, this is when it will occur. If there's a chromosomal problem, some important gene doesn't turn on or do what it's supposed to do and the heart will stop developing."

However, older mothers tend to experience vanishing twin losses more frequently. "Spontaneous fraternal twins are more common in older moms because they ovulate more than one egg more often. And older women have a higher rate of miscarriage because of chromosomal abnormalities," Givens says.

Unlike the miscarriage of a singleton, a woman is less likely to have bleeding with a vanishing twin because the healthy embryo keeps generating hormones to keep the placental lining in place for nourishment and protection. That's why many women have no idea the second twin even existed. There were no obvious signs that it was ever there and no physical indications of a problem when it died. While there is "minimum physical risk to the mother if it happens in the first trimester," according to Dr. Givens, there is a small (less than 10%) chance that the healthy baby will be lost at the same time. In that event, both embryos would be washed away in the blood of what appears to be a heavy period, but is actually an early stage miscarriage.

Late-Term Loss

Tragically, just like some singleton babies are lost in the second or even third trimesters, the same goes for twins. But the risks to the mother and surviving baby are much greater in the case of a multiple pregnancy. When a single baby stops growing, the body either expels the fetus, resulting in a miscarriage or a stillborn baby, or a doctor removes the tissue for the safety of the mother. But when a twin is lost late in a pregnancy a few different things can happen.

"If you lose a twin after 20 weeks, you are at a much higher risk to lose the other one. There's so much tissue present that the body wants to expel it and take the live one with it," Dr Givens explains, "The major risk to losing a twin when the other isn't ready to be delivered, is the prematurity of the surviving twin."