The average weight for Singaporean Chinese babies at 40 weeks is 3.2kg. 10% of babies weigh in excess of 3.6kg, and 3% weigh in excess of 4kg.
Factors that cause larger-than-average babies include genetics (parental height and race), maternal diabetes, previous big baby, pre pregnancy maternal obesity, excessive maternal weight gain and post-date pregnancy. Environmental and genetic interactions are complex. Much birth weight variation is unexplained, and most big infants have no identifiable risk factors. Further no risk factors predict big babies accurately.
One concern is that big babies are more likely to have shoulder dystocia ((shoulders getting stuck) at vaginal delivery, requiring special manoeuvres to deliver the baby. Shoulder dystocia is a cause of labour-related baby deaths and infant nerve injuries. A local study of shoulder dystocia in Singapore determined that a birth weight above 3.6kg was associated with risk of shoulder dystocia that was 16 times higher than during pregnancies resulting in the delivery of an infant weighing less than 3.6kg. Tears involving the anal sphincter, a muscle in the back responsible for the continence of faeces, are increased with shoulder dystocia.
Other risks to the baby during labour include nerve and skeletal injuries during delivery, and lack of oxygen during labour. Risks after delivery for the baby include low sugar levels. Prolonged labour also increases the risk of severe bleeding after birth in caesarean deliveries. Anaesthetic complications are also increased greatly because of the increased need for it during operative deliveries of big babies.
If the mother is carrying a big baby, the decision regarding the safest mode of delivery would require an individualised discussion between the woman and her doctor.