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.
At every antenatal visit, the obstetrician will 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 than it should, given the progression of the pregnancy, the doctor will consider an ultrasound to estimate foetal weight.
Foetal weight can be estimated through an ultrasound by measuring the size of the baby's stomach. Unfortunately, ultrasound is not perfect as the margin of error (over prediction and underestimation) is 10-15%.