Early amniotomy increases the frequency of fetal heart rate abnormalities. Amniotomy Study Group.

OBJECTIVE

To determine whether early amniotomy, when practised as an isolated intervention, increases the hourly rate of fetal heart rate record abnormalities.

DESIGN

This is a secondary analysis of the results of a multicentre randomised trial of early versus late amniotomy in labour.

SETTING

Secondary and tertiary level teaching hospitals.

INTERVENTION: Early amniotomy versus an attempt to conserve the amniotic membranes.

MAIN OUTCOME MEASURES: The hourly rates of early, mild variable, severe variable and late decelerations; caesarean section rates.

RESULTS

Severe variable decelerations, when classified as categorical events (> or = 1/h to 2/h, > or = 2/h to < 4/h, > or = 4/h), were more frequent in the amniotomy group (chi2 for trend = 5.7, P = 0.017). The mean hourly rates of severe variable and late fetal heart rate decelerations were increased in the amniotomy group (severe variable: amniotomy group 1.4/h, control 0.7/h, P = 0.021; late: amniotomy group 3.3/h, control 2.3/h, P = 0.011). Although the overall rate of caesarean was similar in the two groups (OR 1.2; 95% CI 0.8-1.8), there was an increase in caesarean section for fetal distress (OR 2.3; 95% CI 1.1-4.5) associated with amniotomy.

CONCLUSION

Our data suggest that early amniotomy increases the hourly rate of severe variable fetal heart rate decelerations without evidence of an adverse effect on neonatal outcome. In settings where the diagnosis of fetal compromise is based primarily on electronic monitoring, caesarean section for fetal distress may be increased by early amniotomy.