Monozygotic or monochorionic twins (uniovular/monovular/identical)

These are produced when one ovum divides to form two fetuses (Figure 21.1a). They can be dichorionic diamniotic, monochorionic diamniotic or monochorionic monoamniotic. The monozygotic twinning rate of 3–5 per 1000 is similar for all ethnic groups (the fetal sex is always the same). Complications such as hydramnios, fetal abnormality, discrepancy in fetal weight and fetal transfusion syndrome, are more likely, as are intrapartum complications and poor perinatal outcome. When one twin dies in utero there is a 25% risk of neurological and renal lesions and even intrauterine death in the survivor.

Figure 21.1 Monozygotic twins (a) and dizygotic twins (b).

Dizygotic twins (binovular, non-identical)

With dizygotic twins, two ova are shed, usually in the same menstrual cycle and fertilised by two different sperms (Figure 21.1b). The fetal sex can differ. Twinning rate differs in different ethnic groups and is influenced by the use of fertility drugs to induce ovulation.

Other multiples

These are seldom monovular, they usually follow multiple ovulation or combinations of monovular and multiple ovulation.

Frequency of combinations of presentation

First twin

First twin has a higher perinatal mortality than with singleton births because of prematurity, cord prolapse, fetal abnormality and birth injury (double the singleton rates).

Second twin

With the second twin malpositions, malpresentations, prolonged delivery interval, likelihood of placental separation and greater need for operative delivery result in lower Apgar scores. The neonatal mortality, mainly due to birth trauma, is twice that of the first twin. The danger of hypoxia increases the risk of cerebral palsy in the second twin.

In general the perinatal mortality rate of twins is four to five times that of singletons. Mortality due to asphyxia is five times that of singletons. Risk of stillbirth between 37 and 42 weeks is 6–9 per 1000 births. Induction at 38 weeks is advised.