Around 5% of pregnant women in the UK are severely obese, and the scale of the problem is not being met by the correct care, according to the Centre for Maternal and Child Enquiries.
Maternity units are under-prepared to deal with obesity in pregnant women, which is expected to increase as obesity rates rise among the general population, it added.
The centre's three-year study concluded that around one in 20 women who are pregnant have a body mass index (BMI) of 35 or above, categorising them as severely obese. This is the equivalent average of 38,478 women a year.
Wales was found to have the highest rate of severe maternal obesity in the UK, at 6.5%, or one in every 15 pregnant women. In England, the region with the highest rate was East of England at 6.2%, or one in every 16 pregnant women, with London recording the lowest rates at 3.5%, or one in 29.
Stillbirth rates in severely obese women were 8.6 per 1,000 singleton births, twice as high as the overall national stillbirth rate of 3.9, with the risk of stillbirth growing with increasing obesity, the study showed.
Tim Draycott, Clinical Lead at the centre and a Consultant Obstetrician at Southmead Hospital in Bristol, said: "It is difficult to exaggerate how big a problem this is for most of us working in the NHS. There are significant associated complications for mothers and their babies and much of that harm is preventable given the right care."