With most congenital malformations a baby is not large enough, or misshapen enough, to cause difficulty during labour. The important exceptions are anencephaly and hydrocephaly, for which you should use the methods below. If you have the misfortune to find a grossly abnormal conjoined twin, Caesarean section is the method of choice.
Anencephaly is complicated in 90% of cases by polyhydramnios; so when you diagnose this, do an ultrasound on the mother to see if her baby has a head. If he has not, he is usually stillborn, and even when he is born alive, he does not survive more than a few hours. When you have explained the diagnosis to her, she will usually insist that her pregnancy is induced.
Hydrocephalus is not always easy to diagnose clinically, and is often missed during pregnancy. A common mistake is to misdiagnose a brow presentation (when the head feels big) for hydrocephalus. If you suspect it, confirm the diagnosis by ultrasound. If the diagnosis is then obvious, proceed as described below. If it is doubtful, wait. Even during labour the diagnosis is easily missed, if widely distended sutures and fontanelles cannot be felt.