A RISE in the number of obese women becoming pregnant has sparked calls for more vigilant monitoring and weighing of expectant mothers, amid fears babies' lives are being put at risk.

Specialists at leading Melbourne hospitals have told The Sunday Age that women with pre-pregnancy weights of 150 kilograms or more are increasingly common, with some then adding up to 30 kilograms before giving birth — around three times the recommended weight gain of 7 kilograms to 12 kilograms.

The mothers' excess fat is posing serious challenges for medical staff, who are struggling to detect babies on ultrasound machines and monitor their heart rates. Many of the women suffer obesity-related diabetes and high blood pressure, with their size tripling their babies' risk of sudden death or birth defects.

Pregnancies for very large women are considered so risky that most hospitals are turning away expectant mothers with a body mass index higher than 35 or 40. A healthy BMI is 20 to 25.