Physical stress experienced by a fetus due to a lack of oxygen
Fetal distress develops when there is an insufficient oxygen supply reaching the fetus. Although this condition usually occurs during labour or after week 28 of pregnancy, it can develop at any time. Fetal distress occurs in about 1 in 20 pregnancies, but the fetus is usually delivered before lasting harm occurs. However, left untreated, fetal distress may cause fetal brain damage or death.
A common cause of fetal distress in pregnancy is a problem with the placenta, the organ that supplies oxygen and nutrients via the umbilical cord to the fetus. The function of the placenta may be affected if the mother develops pre-eclampsia (see Pre-eclampsia and eclampsia). The oxygen supply to the fetus may also be reduced if part or all of the placenta separates from the uterus during pregnancy (see Placental abruption).
Frequently, the cause of fetal distress during labour is unknown. There may be a placental abruption during labour, or the umbilical cord may become tangled, preventing oxygen from reaching the fetus. Another, less common, cause of fetal distress is cord prolapse. This condition may develop if the fetus is not securely fitted into the mother’s pelvis because it is not lying in the normal head-down position facing the mother’s back (see Abnormal presentation). An abnormal position may create space for the umbilical cord to drop through the cervix into the vagina. Once it is outside the uterus, the blood vessels in the cord may become compressed or, rarely, may go into spasm. Both of these situations can reduce the oxygen supply to the fetus.
The risk of fetal distress is increased if the fetus is weak or smaller than average (see Intra-uterine growth retardation) or if your labour starts before the 37th week of pregnancy (see Premature labour).
If your doctor is concerned that there is a risk of fetal distress during your pregnancy, he or she may decide to refer you for an additional ultrasound scan (see Ultrasound scanning in pregnancy). This scan enables your doctor to observe fetal movements and also to look at the blood flow in the vessels carrying blood from the placenta to the fetus. Reduced fetal movements may indicate oxygen deficiency. Your doctor may also recommend fetal monitoring. This monitors the fetus’s heart rate, which may be abnormal if the fetus is not getting enough oxygen.
During labour, the fetal heartbeat is continuously monitored, and a sample of blood may be taken from the fetus’s scalp to measure blood oxygen levels. The amniotic fluid from the uterus may be examined regularly for signs of staining from fetal faeces (meconium), which are sometimes released from the fetus when it is under stress.
Treatment for fetal distress depends on the cause, the severity of the distress, whether it occurs before or in labour, and how far labour has progressed.
If there are signs of even mild fetal distress during late pregnancy or early labour, your doctor will probably recommend a caesarean section. If labour is advanced and the fetus is experiencing only mild levels of distress, an assisted delivery may be a possible option. However, if fetal distress is severe, or there is a cord prolapse, your doctor will probably advise an emergency caesarean section.
If fetal distress is diagnosed and treated promptly, there is a good chance that your baby will be delivered safely and with no permanent harm. However, in some situations – for example, when a baby is premature (see Problems of the premature baby) – your baby will need careful monitoring after birth (see Special care baby unit).
From the 2010 revision of the Complete Home Medical Guide © Dorling Kindersley Limited.