Around one in 20 pregnant women in the UK is severely obese, putting themselves and their babies at a greater risk of a series of health problems, according to a study published today.
Maternity units are "under prepared" for the scale of the obesity problem amongst pregnant women which is expected to increase as obesity grows in the general population, the Centre for Maternal and Child Enquiries (CMACE) said.
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A three-year study by the centre for UK maternity units has shown around 5% of pregnant women, or one in 20, have a body mass index (BMI) of 35 or more, putting them into the category of severely obese - a total of around 38,478 every year.
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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.
Obesity in pregnancy is associated with an increased risk of a range of health problems and "serious adverse outcomes" the report authors said, including miscarriage, diabetes, high blood pressure, potentially fatal blood clots, haemorrhage, still birth and neonatal death.
Women who are obese also have a higher caesarian rate and a lower breastfeeding rate compared to women with a healthy BMI, and have an increased risk of having a baby with a congenital malformation, the report found.
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.
Pregnant women, and especially obese pregnant women, are more at risk of developing venous thromboembolism (VTE), a potentially fatal condition when a blood clot forms in a vein which may break away and block a blood vessel, the study noted.
But the risk of VTE was "poorly documented" for obese pregnant women at their first antenatal appointment and the study found fewer than 50% at moderate or high risk of VTE were offered treatment to prevent the condition.
For those women who did receive treatment antenatally, the prescribed doses were considered to be "insufficient" for their body weight, according to guidelines published by the Royal College of Obstetricians and Gynaecologists (RCOG).
The CMACE study also identified "gaps" in anaesthetic care for obese women.
Women should be supported to lose weight before conception and should be provided with "good" information and advice on the risks associated with obesity during pregnancy and childbirth, the study concluded.
The BMI calculation is reached by dividing weight in kilogrammes by the square of a person's height in metres.
A person is classified as obese if their BMI is 30 or greater.
The report included a study of 6,413 women with a BMI of 35 and over who gave birth in the UK during March and April last year.
Researchers also studied a clinical audit of maternity care received by 905 women with a BMI of 35 or over.
Dr Imogen Stephens, CMACE clinical director, said: "This CMACE report shows that much more needs to be done in the NHS to deal with the growing numbers of obese pregnant women.
"We have already shown in our previous survey how specialist equipment such as wheelchairs, trolleys and beds are needed to care for this unique group of women.
"The findings from this new study show that the risks of clinical intervention increase with increasing levels of obesity and that specialist obstetric care is needed. All this requires improved, and better integrated, care for these women."
Tim Draycott, clinical lead at CMACE 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."