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 usually recognized very early on. Serial ultrasound follow-up examinations in such cases have shown that often times one of the developing babies “mysteriously” vanishes, with the remaining conceptus (baby) usually proceeding to a healthy birth. Since most multiples comprise twin pregnancies, the term “vanishing twin” is used to describe this situation. While in most cases the loss of a vanishing twin is associated with painless, innocuous bleeding, this is not always the case. In fact bleeding might not occur at all.

The incidence of spontaneous pregnancies resulting in twin births is about 1:80. In women under 35 treated with clomiphene citrate it is about 1:20. After ovarian stimulation using injectible gonadotropin fertility medications, it increases to approximately 1:5 and after IVF (given the current tendency to replace multiple embryos), the incidence of twins is about 1:3.

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. When this happens, the occurence of painless mild bleeding (or spotting) raises understandable concerns by the affected woman who often asks:

Q. – Am I about to miscarry?

Answ: 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 accordingly usually not affect the remaining twin. As long as the bleeding remains mild and the woman 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 this is precisely what happens.

Q. How long will I continue to bleed?

Answ: In most cases – unless the pregnancy is destined to miscarry completely – the bleeding will remain painless, mild, and will stop within a week or so. However, the timing will depend on when the conceptus was lost. If this occurred late in the first trimester, the bleeding will usually last longer (even a few weeks) than in cases where the pregnancy was lost earlier. It should be that in some cases , and the body reabsorbs one twin with no outward indication (bleeding) of the loss.

Q. How will the loss of one twin affect the surviving one?

Answ: In the majority of cases, the remaining conceptus will progress unaffected, to a healthy birth.

Q. – Will there be any remaining evidence of the vanished twin at birth?

Answ: Usually not! Sometimes a small area of scarring or “thickening” of part of the placenta will be seen at birth. However, this usually only occurs in cases where the first twin succumbed late in the first trimester or beyond (in the 2nd trimester).

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

Answ: The vanishing twin is not the result of something the mother/father did – or failed to do. In most cases, the vanishing twin is lost for the same reason that single pregnancies miscarry (i.e. because af chromosoma aneuploidy). In a small number of cases the loss results from autoimmune or alloimmune implantation dysfunction. Since many vanishing twins are lost very early in pregnancy, before most women will have undergone an ultrasound to confirm pregnancy, most cases go undetected. The woman would have no knowledge that she had been carrying more than one fetus. In fact, as stated above, many more of us begin life as twins than was previously thought.

Vanishing concepti can also occur within high order multiple pregnancies (triplets or greater). A triplet pregnancy can reduce spontaneously to a twin or singleton and so can a quadruplet pregnancy.

In the final analysis, individuals and families who experience the “vanishing” of a conceptus will experience anxiety and even panic when bleeding starts, followed by a sense of relief when it finally stops, and they learn that the remaining fetus and the pregnancy have survived. This will in most such cases be followed by a profound sense of loss and sadness , especially if the woman and/or her partner had been looking forward to a twin birth.