Betamimetic tocolytics are drugs that reduce contractions and delay labour, which can improve placental blood flow and fetal oxygenation. They may be suggested to a woman during labour if the baby is showing signs of stress, such as an unusual heartbeat. Tocolysis may also be used to prevent fetal distress (prophylactically) during the second stage of labour, the time from when the cervix is fully dilated by the baby’s head to actual childbirth.
The review authors searched the medical literature for randomised controlled trials comparing prophylactic intravenous betamimetic therapy with inactive or no treatment for women with uncomplicated pregnancies and whose babies were not showing signs of stress during the second stage of labour. They identified two trials involving 164 women, both conducted in the 1970s. One trial reported on clinical outcomes and found no beneficial differences in clinical neonatal outcome or Apgar scores at two minutes with tocolysis. The mean umbilical arterial pH values were slightly higher in the treatment groups of the two trials and intravenous betamimetics may prevent the deterioration of fetal arterial pH levels during the second stage of labour, possibly related to use of the supine position in these trials. Women receiving betamimetic drugs were more likely to have forceps delivery, which is the use of surgical tongs to assist the baby through the birth canal. Both trial protocols required forceps to be used if the second stage of labour, or time from initiation of the betamimetic infusion, exceeded 30 minutes. There were no clear differences in postpartum haemorrhage. The authors found inadequate evidence of benefit to recommend the prophylactic use of tocolytics in the second stage of labour.