What is the issue?
Babies who are very large (or macrosomic, weighing over 4000 g when born) can have difficult and occasionally traumatic births. One suggestion to try to reduce this trauma has been to induce labour early, before the baby grows too big. The estimation of the baby's weight is difficult before birth and not very accurate. Clinical estimations are based on feeling the uterus and measuring the height of the fundus of the uterus. Both are subject to considerable variation. Ultrasound scanning is also not accurate, so suspected large babies may not be confirmed at delivery. This may worry parents.
Why is this important?
If undertaken too early, induction of labour can lead to babies being born prematurely and with immature organs.
What evidence did we find?
We found four trials that assessed induction of labour at 37 to 40 weeks for women when it was suspected that their baby was large. A total of 1190 pregnant, non-diabetic women were involved. We searched for evidence on 31 October 2015. The studies were of moderate or good quality although it was not possible to blind the women and staff providing care to which group women had been assigned. This may have introduced bias.
What does this mean?
The number of births where the baby's shoulder became stuck (shoulder dystocia) or a bone was fractured (usually the clavicle, which heals well without consequences) were reduced in the induction of labour group. The evidence was assessed as moderate quality for shoulder dystocia and high quality for fracture. No clear differences between groups were reported for damage to the network of nerves that send signals from the spine to the shoulder, arm and hand (brachial plexus injury) of the baby (low-quality evidence due to very few events occurring) or signs of not enough oxygen during birth. A policy of labour induction reduced the average birthweight of babies by 178 g. The trials did not show any differences in the number of women who had caesarean sections or instrumental births. There is limited evidence that more women in the induction of labour group had severe damage to the perineum. We conclude that there appear to be benefits, but there may also be some disadvantages of induction of labour shortly before term. The option of having an induction should be discussed with parents when their baby is suspected to be extra large.
Although some parents and doctors may feel the existing evidence is sufficient to justify inducing labour, others may disagree. Further high-quality studies are needed in order to find out what is the best time to induce labour towards the end of pregnancy, and how to improve the accuracy in diagnosing macrosomia.
Induction of labour for suspected fetal macrosomia has not been shown to alter the risk of brachial plexus injury, but the power of the included studies to show a difference for such a rare event is limited. Also antenatal estimates of fetal weight are often inaccurate so many women may be worried unnecessarily, and many inductions may not be needed. Nevertheless, induction of labour for suspected fetal macrosomia results in a lower mean birthweight, and fewer birth fractures and shoulder dystocia. The unexpected observation in the induction group of increased perineal damage, and the plausible, but of uncertain significance, observation of increased use of phototherapy, both in the largest trial, should also be kept in mind.
Findings from trials included in the review suggest that to prevent one fracture it would be necessary to induce labour in 60 women. Since induction of labour does not appear to alter the rate of caesarean delivery or instrumental delivery, it is likely to be popular with many women. In settings where obstetricians can be reasonably confident about their scan assessment of fetal weight, the advantages and disadvantages of induction at or near term for fetuses suspected of being macrosomic should be discussed with parents.
Although some parents and doctors may feel the evidence already justifies induction, others may justifiably disagree. Further trials of induction shortly before term for suspected fetal macrosomia are needed. Such trials should concentrate on refining the optimum gestation of induction, and improving the accuracy of the diagnosis of macrosomia.
Women with a suspected large-for-dates fetus or a fetus with suspected macrosomia (birthweight greater than 4000 g) are at risk of operative birth or caesarean section. The baby is also at increased risk of shoulder dystocia and trauma, in particular fractures and brachial plexus injury. Induction of labour may reduce these risks by decreasing the birthweight, but may also lead to longer labours and an increased risk of caesarean section.
To assess the effects of a policy of labour induction at or shortly before term (37 to 40 weeks) for suspected fetal macrosomia on the way of giving birth and maternal or perinatal morbidity.
We searched the Cochrane Pregnancy and Childbirth Group's Trials Register (31 January 2016), contacted trial authors and searched reference lists of retrieved studies.
Randomised trials of induction of labour for suspected fetal macrosomia.
Review authors independently assessed trials for inclusion and risk of bias, extracted data and checked them for accuracy. We contacted study authors for additional information. For key outcomes the quality of the evidence was assessed using the GRADE approach.
We included four trials, involving 1190 women. It was not possible to blind women and staff to the intervention, but for other 'Risk of bias' domains these studies were assessed as being at low or unclear risk of bias.
Compared to expectant management, there was no clear effect of induction of labour for suspected macrosomia on the risk of caesarean section (risk ratio (RR) 0.91, 95% confidence interval (CI) 0.76 to 1.09; 1190 women; four trials, moderate-quality evidence) or instrumental delivery (RR 0.86, 95% CI 0.65 to 1.13; 1190 women; four trials, low-quality evidence). Shoulder dystocia (RR 0.60, 95% CI 0.37 to 0.98; 1190 women; four trials, moderate-quality evidence), and fracture (any) (RR 0.20, 95% CI 0.05 to 0.79; 1190 women; four studies, high-quality evidence) were reduced in the induction of labour group. There were no clear differences between groups for brachial plexus injury (two events were reported in the control group in one trial, low-quality evidence). There was no strong evidence of any difference between groups for measures of neonatal asphyxia; low five-minute infant Apgar scores (less than seven) or low arterial cord blood pH (RR 1.51, 95% CI 0.25 to 9.02; 858 infants; two trials, low-quality evidence; and, RR 1.01, 95% CI 0.46 to 2.22; 818 infants; one trial, moderate-quality evidence, respectively).
Mean birthweight was lower in the induction group, but there was considerable heterogeneity between studies for this outcome (mean difference (MD) -178.03 g, 95% CI -315.26 to -40.81; 1190 infants; four studies; I2 = 89%). In one study with data for 818 women, third- and fourth-degree perineal tears were increased in the induction group (RR 3.70, 95% CI 1.04 to 13.17).
For outcomes assessed using GRADE, we based our downgrading decisions on high risk of bias from lack of blinding and imprecision of effect estimates.