Until the last part of the twentieth century, Hellin's Law governed the predictability of multiple births – the natural occurrence of twins in the general population is 1/100, and the frequency of each higher multiple is determinable by multiplying the denominator by 100, so that the frequency of triplets is 1/10 000, the frequency of quadruplets is 1/1 000 000, and so on. Since the advent of fertility drugs in the 1960s and in vitro fertilization (IVF) in the 1970s, the incidence of multiple gestations has increased markedly. By the late 1980s, the rate of multiple births had more than tripled; it appears to be rising still (Hammon, 1998: p. 338).

With each higher order of multiples, risks to both fetus and pregnant woman escalate. For women, the risks include anaemia, preterm labour, hypertension, thrombophlebitis, preterm delivery and haemorrhage. Tocolytic therapy to avoid preterm delivery introduces further risks. For fetuses or potential children, the risks include intrauterine growth retardation, malpresentation, cord accidents and the usual sequelae of preterm delivery, such as respiratory distress, intracranial haemorrhage and cerebral palsy (Hammon, 1998: p. 339).

Conflicts between the interests of pregnant women and their fetuses are not new; attempts to induce abortion and to rescue fetuses have occurred through most of human history. Although medical advances have considerably reduced the mortality and morbidity risks of childbearing for most women and their offspring, that same technology has introduced methods by which people who would not otherwise reproduce can have biologically related children.