There is a golden rule in determining zygosity; that if there is one placenta and two babies, then we have identical twins. If the babies grow up to look a bit different from each other, the idea of their being identical still seems to remain.
On the other hand, if we have two placentas, do we have fraternal twins? A percentage of identical twins do develop separate placentas, so we cannot use the number of placentas as a certain guide. At an individual level, pairs of twins can seek out an expert to prove their zygosity by looking directly at their genes.
A further complication is the “vanishing twin” pregnancy. A pair of fraternal twins may be a triplet set, reduced to twins in the earliest stages of pregnancy and not picked up by ultrasound. If the lost triplet was of the opposite sex and an identical pair survived, then the transfer of gender-related hormones, which is known to occur between fraternal twins, may obscure the results if this twin pair takes part in research. Equally, if the lost triplet was one of an identical pair, then the surviving fraternal twins will not simply be the result of a double ovulation but instead will be a reduced triplet pregnancy, resulting in one surviving identical twin and another who is genetically unrelated. When one considers the complexities of naturally reduced triplet sets, then the placenta rule, which is after all no more than a rule of thumb, becomes far too unreliable.
When a twin dies before birth, the sole survivor needs help and understanding. Womb twin survivors are the sole survivors of a twin or multiple pregnancy. This group, 1 in 10 of the population, includes survivors of a stillbirth, miscarriage, abortion and a "vanishing twin" pregnancy. It is a story of a twin bond broken by death, leaving a lonely survivor.
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