Midwives not measuring expectant mums for fear of offending them, researchers find.
The height and weight of pregnant women is commonly misreported when they book in for maternity care, putting them and their babies at risk, research shows.
Fear of offending women has emerged as a reason some aren't asked to stand on the scales, despite guidelines recommending that every pregnant woman be weighed.
A study of 248 pregnant women found nearly two-thirds were heavier than their recorded weight. Fifteen per cent were lighter, and for only 23 per cent the figure was about right - within 0.5kg.
The recordings were made on a laboratory order form for Down syndrome screening tests, routinely offered between the ninth and 13th weeks of pregnancy.
"When completing the 'booking form' at the beginning of pregnancy, there is evidence to suggest that women are being asked to report their weight and height, rather than being measured," obstetrician-gynaecologist Dr Helen Paterson and her Otago University colleagues say in a paper on their research in today's NZ Medical Journal.
"Self-reported data tend to underestimate weight and overestimate height."
They say incorrect recordings of a woman's body mass index, for instance reporting her as overweight when she was really obese, could lead to a high-risk pregnancy not being recognised - and consequently she might not be referred to specialist care when she should.
One midwife quoted in the study said: "It's a really sensitive issue for me ... I have a lot of trouble talking to women about their weight ... the concept of weighing a woman each time I see her. If she's a normal weight then I probably wouldn't."
Maternal obesity is linked to many complications, including pre-eclampsia, gestational diabetes, premature birth, caesarean delivery, infection and haemorrhage.
In 2010, around half of the mothers of babies who died around the time of birth were overweight or obese. "Thus it is important that such high-risk women are identified and referred appropriately."
A further risk of misrecording relates to testing for Down syndrome.
Women for whom Down screening identifies a high risk of having a baby with the syndrome can be offered diagnostic testing with amniocentesis, which uses a sample of the fluid surrounding the fetus. The procedure carries a 1 per cent risk of losing the pregnancy.
If a woman's weight is reported falsely low, she may be judged at high risk of having a Down syndrome baby - and be offered amniocentesis - when, based on her real, higher weight, she is in fact at low risk.
Leaders of the College of Midwives were unavailable to comment.
• Study of 248 pregnant women in Christchurch in 2011
• 62% heavier than the weight in their medical records.
• 44% shorter than recorded height.
• 69% body mass index higher than recorded BMI.
• 17% placed in wrong BMI category, such as "normal'' instead of "overweight''.
Source: NZ Medical Journal.