What is the issue?

Excessively strong or frequent contractions can occur in any labour, though are more common when women have been given medications to start off or increase contractions. In some cases, excessive contractions can be a sign of complications such as placental abruption or obstructed labour. Excessive contractions may reduce the amount of oxygen reaching the unborn baby.

‘Tocolysis’ is when women are given medication to reduce the strength or frequency of contractions, or both. Tocolysis may improve blood flow and therefore improve the baby’s well-being. This review aimed to evaluate the benefits and harms of tocolysis when the uterus (womb) is contracting too quickly (more than 5 contractions in 2 consecutive 10-minute periods) or when the baby is showing signs of distress during labour, as detected by monitoring its heart rate.

This new Cochrane Review supersedes an earlier Cochrane Review with the same name.

Why is this important?

Babies who are deprived of oxygen during labour can develop serious problems, including cerebral palsy, organ damage or death. When fetal monitoring suggests fetal distress, measures can be taken to improve the baby’s oxygen levels. This can include the use of tocolytic medications. This may be particularly important in low-resource environments, where an emergency delivery or caesarean section may not be immediately available.

What evidence did we find?

We searched for evidence in February 2018 and found eight randomised controlled trials (involving 734 women) who had excessive uterine contractions, signs of fetal distress, or both during labour. The trials tested different comparison groups, which means that our analyses were based on data from single studies involving small numbers of women. Women were randomised to receiving a β₂-adrenergic tocolytic drug or an alternative approach (including no tocolytic while awaiting caesarean section, stopping medications that increase contraction strength, or using a different tocolytic such as atosiban, magnesium sulphate or nitroglycerin).

We combined data from two small trials (57 women), comparing a β₂-adrenergic tocolytic drug with no tocolytic drug. Two babies died in their mother’s womb, both occurring in the group of women who did not receive a tocolytic – one had gross hydrocephalus (too much fluid in and around the brain) and the other occurred whilst the mother was waiting to have a caesarean section. The number of babies with an abnormal fetal heart rate is probably lower in the group of women who were given a tocolytic, but the effects on other measures of infant well-being were uncertain.

Very few serious side effects were found, but the studies were too small to assess uncommon adverse effects.

It is not possible to draw clear conclusions about the benefits and harms and the quality of the evidence was very low to moderate.

What does this mean?

There is not enough evidence from randomised controlled trials to determine the effects of tocolysis for women with fetal distress or excessive uterine contractions, nor to identify whether one type of tocolytic drug is safer or more effective than another.

The clinical significance for some of improvements in measures of fetal well-being with tocolytics is unclear. Generally, sample sizes were too small to detect effects on maternal or infant well-being or serious adverse effects. The majority of studies were from high-income countries in healthcare facilities with access to caesarean section, which may limit the applicability of these results to lower-resource settings, or settings where caesarean section is not available.

Further high-quality studies, involving large numbers of women, are needed. Such studies could focus on measuring clinically relevant outcomes for the mother and her baby such as death of the mother, her baby, and other measures of well-being and safety.