Humans do not normally produce litters. Nevertheless,it is estimated that one person in 20 who was born alone has a lost twin who was conceived at the same time,but failed to reach term.
It has been known for many years that the loss of one twin in this way late in a pregnancy is bad for the other. That other is more likely to be born prematurely,to have cerebral palsy,or even to die as well. What has not been known until now is whether such a loss in the first few months has any effect on the survivor.
But a paper just published in Human Reproduction by Peter Pharoah of the University of Liverpool,in Britain,suggests that it does.
There are several ways that a vanished twin can come to light. Most gruesomely,fully formed body parts of the dead twin may be found embedded in the body of its surviving sibling. More commonly,lost twins are discovered as tiny mummified attachments to the placenta of the live twin,after it is born. And the widespread use of ultrasonic scanning means that,increasingly often,twins are spotted in early scans and subsequently vanish. Neither early ultrasonic sightings nor papery mummified corpses are always officially registered. They may not even be mentioned to the mother,so it is hard to know just how often a twin goes missing.
Dr Pharoah,however,made a stab at finding out. He examined three sets of data from northern England. One collated information about infant deaths. A second recorded congenital abnormalities. The third was a register of all pregnancies of twins,triplets or other multiple births.
What made this last register so useful was that it recorded multiple pregnancies as soon as they were recognised ultrasonically,rather than at birth. That made it possible to identify which babies born as singletons had actually started their lives as twins. It also,when possible,noted whether foetuses were surrounded by their own chorionic membrane,or whether that chorion was shared.
Between 1998 and 2004 some 213,087 babies were born in the region covered by the register. The rate of congenital anomalies was 1.6 times higher for twins than for singletons.
But when Dr Pharoah looked at what happened to foetuses who had survived the early death of their co-twin,the numbers got worse.
He found 138 instances in which one twin died before 16 weeks gestation and the other was born and survived. Eleven of these survivors had a congenital anomaly such as a malformation of the heart or a facial cleft. From these figures Dr Pharoah calculates that the risk of congenital anomaly in a surviving twin following the early loss of its sibling is 2.4 times higher than if the sibling survives — and almost four times higher than that of true singletons,who had the womb all to themselves from day one. He thinks the harm is done if the blood supply is shared between the two individuals,something more common in twins who share a chorion.
Whether this knowledge can be used to help surviving twins is moot. But identifying a problem is the first step to remedying it. Dr Pharoah seems to have done that.