The estimated foetal weight is calculated through ultrasound, measuring the size of the baby's abdomen. However, it’s important to note that ultrasound estimations are not infallible, with an error margin of 10–15% in overestimation or underestimation.

Vaginal birth is generally encouraged for all women, barring specific medical indications for a caesarean section. A caesarean section, being a major surgery, presents greater risks than a vaginal birth, including potential complications like infection, the necessity for blood transfusions due to bleeding and the risk of blood clots in the legs. In addition, future births are also impacted, with increasing risks of scar rupture during labour in subsequent vaginal births and heightened surgical complications in subsequent carsarean sections.

A significant concern with big babies is the increased likelihood of shoulder dystocia during vaginal delivery, where the baby’s shoulders become lodged, requiring specialised manoeuvres for delivery. Shoulder dystocia is linked to labour-related infant fatalities and nerve injuries. A local Singaporean study found that a birth weight above 3.6 kg is associated with a 16-fold increase in the risk of shoulder dystocia compared to infants weighing less than 3.6 kg. Therefore, when a mother is carrying a big baby, the decision regarding the safest delivery method should be made following an individualised consultation between the woman and her doctor.

Several factors contribute to foetal size, including genetics (parental height and race), maternal diabetes, a history of delivering big babies, pre-pregnancy maternal obesity, excessive maternal weight gain and post-term pregnancy. The interaction between environmental and genetic factors is complex. Most big babies have no identifiable risk factors and no single factor accurately predicts a large birth weight. 

A healthy weight gain for a woman in her first pregnancy, without dietary restriction, typically ranges from 10 to 16kg. There is a notable correlation between maternal weight gain and the baby's birth weight. Recent studies from the United States indicate that excessive weight gain (over 18kg) can double the risk of delivering a big baby.