Obesity in pregnancy

It is a Tuesday booking clinic. A 33-year-old G2P1 walks in at nine weeks. She is 168 cm tall and her scale weight reads 102 kg — BMI 36. She has been on combined oral contraceptive until last month, has polycystic ovary syndrome and her mother has type 2 diabetes. If you book her as routine antenatal care, you have missed every risk that the pregnancy carries. Obesity in pregnancy is the commonest modifiable risk factor on a labour ward, and almost every fellow-level question on this topic tests whether you have tiered the plan to the BMI.[1][4]

Overview and definition

Maternal obesity is BMI 30 or above calculated from a measured weight and height in early pregnancy. The deeper point is that obesity is not a single risk — it is a multiplier of every obstetric complication, and the multiplier scales with the BMI class.[1][2]

• WHO BMI tiers drive the tiered antenatal plan: overweight 25 to 29.9, class I obesity 30 to 34.9, class II 35 to 39.9, class III (morbid) 40 and above.[1][2]
• Three BMI thresholds, three actions — BMI 30 is the screening cut, BMI 35 is the anaesthetic-referral cut, BMI 40 is the MDT-and-bariatric-equipment cut. Memorise these as triggers, not trivia.
• Asian populations carry equivalent risk at lower BMI — the WHO Asian cutoffs (overweight at 23, obese at 25) are clinically relevant in ANZ.[1]
• Gestational weight gain in obese women is targeted at 5 to 9 kg (Institute of Medicine 2009, RANZCOG-aligned).[2]