Today, in first world environments where there is ready access to advanced medical technology, many women undergo ultrasound diagnosis of pregnancy as early as 5-6 weeks after their last menstrual period. As a result, multiple pregnancies are often recognized very early on. Serial ultrasound follow-up examinations performed in such cases have shown that often times one of the developing babies subsequently “mysteriously” vanishes while the remaining, surviving conceptus (baby) proceeds to a healthy birth. Since most multiple gestations are comprised of twin pregnancies, the term “vanishing twin” has emerged as the term to describe this situation.

While in most cases the vanishing of a twin is associated with an innocuous small bleed, this is not invariably the case. In fact, in many cases, the disappearance of the conceptus goes undetected without there being any bleeding or other symptoms at all. The incidence of spontaneous pregnancies resulting in twin births is about 1:80. But what many fail to appreciate is that about 1 in 10 spontaneous pregnancies start off as twins, but as the pregnancy advances into the 1st trimester and beyond, one twin will “vanish” (absorb) while the other will continue as a healthy, unaffected singleton.

The incidence of multiple pregnancies increases in women with absent or dysfunctional ovulation, who take fertility drugs. In fact, the incidence of multiple pregnancies in such women under 35 years of age who are treated with Clomid/Serophene and Femara is about 1:20. In similar women using injectible gonadotropin fertility medications, the incidence is about 1:5. In women (regardless of their ovulatory status) who receive fertility drugs in preparation for IVF (where multiple embryos are usually transferred), the current incidence is about 1:3/1:4.

With a multiple pregnancy, the occurrence of painless mild bleeding, followed by ultrasound evidence that one conceptus is “absorbing” or “vanishing” often evokes understandable alarm and raises several concerns/questions:

1. Q. Am I destined to lose both concepti and miscarry?

Answer: The bleeding results from the absorption of one of the pregnancies, and since the vast majority of twin pregnancies have separate and independent placentas, the loss of one will usually not affect the remaining twin. As long as the bleeding remains mild and she does not experience an increase in cramping and pain over a period of a few days, the pregnancy will probably not be lost. In fact, in the majority of such cases, the other conceptus progresses unaffected to a healthy birth.

2. Q. How long will I continue to bleed?

Answer: In most cases (unless the remaining conceptus is also abnormal and thus destined to miscarry), the bleeding will remain slight, painless, and will usually stop within a week or so. However, this will depend on when the twin succumbed. If it occurred late in the first trimester, the bleeding could last longer (even for a few weeks) than when the pregnancy is lost earlier. It should be borne in mind however, that the loss of one conceptus (the “vanishing twin”) might not be accompanied by any bleeding all. They might absorb one twin without symptoms and with no outward indication of the loss.

3. Q. Will there be any residual evidence of the “vanished twin” detected at birth?

Answer: Usually not! Sometimes a small area of scarring or “thickening” of part of the placenta will be seen. However this will usually only occur in cases where the one conceptus succumbed late in the first trimester (10-12 weeks) or in the 2nd trimester. In rare cases where a baby is lost later in pregnancy, it might not absorb completely and will be expelled with birth of the surviving baby (“fetus papyraceus”).

4. Q. Could the “vanishing” of one twin have been prevented? Did I do something wrong?

Answer: The vanishing twin is not caused by something the mother did (or didn’t do). In most cases, the conceptus is lost because it is chromosomally or genetically abnormal. Since many “vanishing twins” are lost very early in pregnancy – before most women would have undergone a diagnostic ultrasound – most cases go undetected, and the woman would have no knowledge that she had been carrying more than one conceptus. In fact, as stated above, many more of us begin life as twins than was previously thought.

“Vanishing” concepti can also occur in high order multiple pregnancies (triplets or greater). In such cases, the pregnancy could reduce from triplets to twins or even a singleton (or even from a higher number downward). Since the incidence of multiple pregnancies is significantly higher after IVF, where several embryos are often deliberately transferred to the uterus, the incidence of a conceptus “vanishing” is greatest with IVF.

Regardless of the genesis of multiple gestations, individuals and families who experience the “vanishing” of a conceptus will often experience anxiety and even panic when bleeding starts and a sense of relief when it finally stops and they learn that the remaining conceptus and the pregnancy have survived. However, in most cases there will inevitably be an accompanying sense of profound loss, sadness and even grief, especially in cases where prior ultrasound examination had spelled the “promise” of a multiple birth.