Over the past 50 years, published studies have provided quantitative data on the control of epileptic seizures during pregnancy. The studies have varied in quality, and particularly in the ways in which seizure control has been assessed. However, most studies have shown that seizure occurrence rates are more likely to worsen than improve during pregnancy, though in most pregnancies the rates have been unaltered. Nearly all of the studies have involved women with antiseizure medication-treated epilepsy, but there is a little evidence that seizure control also tends to worsen in pregnancies of women with untreated epilepsy. The factors likely to contribute to the seizure worsening are (i) patient non-compliance, (ii) increased antiseizure medication clearance during pregnancy resulting in lower circulating drug concentrations relative to dose, (iii) the effects of the higher female sex hormone levels during pregnancy, oestrogens being pro-epileptogenic and progesterone anti-epileptogenic, and (iv) reluctance to use the potential teratogen valproate in women capable of pregnancy, depriving them of the most effective drug for certain types of epilepsy. Compliance can be encouraged, but at the present time only one other factor is readily correctable, i.e. the increased drug clearance. This can be compensated for by raising antiseizure medication dosage during pregnancy, guided by measurement of circulating drug concentrations. This course of action appears to reduce the chance of seizure disorder worsening during pregnancy, but so far it has not provided a complete solution to the issue.