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.
Fetal weight can be estimated on ultrasound by measuring the size of the baby's tummy. Unfortunately, ultrasound is not perfect as the error margin (over prediction and under estimation) is 10-15%.
Every woman should be encouraged to give birth vaginally, unless there are medical reasons to warrant a caesarean section. Caesarean section is a major operation with greater risks than a vaginal birth. Common risks include infection, bleeding requiring transfusion, and blood clots in the legs. In addition, there are implications for future births. Subsequent vaginal births carry a small risk of scar rupture during labor, and subsequent caesarean sections carry a higher risk of surgical complications
One concern is that big babies are more likely to have shoulder dystocia at vaginal delivery (shoulders getting stuck, requiring special maneuvers to deliver the shoulders.)Shoulder dystocia is a cause of labor related baby deaths and baby's nerve injuries. A local study of shoulder dystocia in Singapore determined that a birth weight above 3.6kg was associated with a 16 times higher risk for shoulder dystocia compared to pregnancies resulting in the delivery of an infant weighing less than 3.6kg. If the mother is carrying a big baby, the decision for the safest mode of delivery would require an individualised discussion between the woman and her doctor.
At every antenatal visit, the obstetrician would palpate the pregnant woman's abdomen, measuring the fundal height measurement, a measurement from the top of the womb to the top of the pubic bone, approximately equivalent to the number of weeks in pregnancy gestation. The estimation is crude, and if on palpation the abdomen feels larger for dates, the doctor would then consider an ultrasound to estimate fetal weight.
Published local data in the 1970s revealed an average birth weight of 3.18kg for Singaporean Chinese babies at 40 weeks, with 10% of babies weighing more than 3.71kg. Unpublished recent data showed that that the average weight for a Singaporean baby is 3.2kg.
Factors included genetics (parental height and race), maternal diabetes, previous big baby, pre pregnancy maternal obesity, excessive maternal weight gain and post-dates 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.
(Footnote: The following risks quoted are for mothers with big babies in general, and does not pertain to Asian mothers alone.)
The total weight gain of a healthy woman in her first pregnancy without dietary restriction ranges from 10 to 16kg. There is a direct link between weight gain of the mother and baby's birth weight. Recent US research shows that excessive weight gain (>18kg) doubles the risk of big babies.