Parturients may require anaesthesia during their pregnancy in order to facilitate obstetric or non-obstetric surgical procedures. Common obstetric procedures required in pregnancy include cervical cerclage, ovarian cyst interventions and fetal surgery. The incidence of incidental or non-obstetric anaesthesia in pregnancy has been reported as ranging from 1:50–150 pregnancies, with appendectomy, cholecystectomy and pelvic laparoscopy being the most commonly encountered procedures.
Incidental anaesthesia must minimize the risk of interrupting the pregnancy and affecting fetal development. Alterations in maternal physiology accentuate anaesthetic risks and the approach to anaesthesia must be modified accordingly.
Anaesthesia may impact on the fetus in two ways: it may interrupt the pregnancy, resulting in either pregnancy loss or premature delivery, or it may impact on fetal development, either by teratogenic pharmacological effects or because of derangements in the physiological milieu.
The effects of a teratogenic insult depend on the gestation of the fetus at the time of exposure. Teratogens cause an ‘all or nothing’ effect in the early pluripotential developmental phase, such that the embryo will either die or be unaffected and develop normally. However, days 31 to 71 of human gestation correspond to the organogenesis phase, where exposure to teratogens may impair the structural development of organ systems. First trimester anaesthesia has been linked with increased neonatal central nervous system and eye malformation, particularly hydrocephalus and cataracts. In the phase following organogenesis, teratogen exposure is more likely to affect the fetal central nervous system, which undergoes development and maturation throughout pregnancy. These effects are likely to result in functional behavioural effects rather than structural malformations.
There is theoretical reason to suspect that drugs used in anaesthesia may be teratogenic: the cellular mechanisms for differentiation and organogenesis include mechanisms known to be targeted by drugs used in anaesthesia. Neurotransmitter and receptor mechanisms, as well as signalling and second messenger systems are all known to be affected by drugs used in anaesthesia. This theoretical risk of teratogenicity has been difficult to quantify in a practical clinical context, because of difficulties extrapolating animal research data to human development (teratogenicity is species specific) and the obvious ethical and logistical problems of research on human fetal exposure to teratogens.