'The increased risk was fairly modest for obese women who did not have conditions such as high blood pressure, diabetes or a previous caesarean section, and the risks were quite low if the woman had given birth previously,' says lead researcher Dr Jennifer Hollowell of the National Perinatal Epidemiology Unit at Oxford University.
'It is important to appreciate that we are not saying that obesity isn't important or that obesity doesn't increase a woman's risks during pregnancy,' she adds. 'We found that around half of very obese women giving birth in obstetric units have medical problems or pregnancy complications when admitted. But our study focused on women who were obese but otherwise healthy when they went into labour, and some of them had much lower risks than might have been expected.'
The Oxford University researchers point out that, among healthy women with a straightforward pregnancy, childbirth risks are influenced more by whether someone is a first-time mum than whether they are obese.
They found that the chances of first-time mums of normal weight having medical interventions or complications during childbirth are greater than for 'very obese' but otherwise healthy women having a second or subsequent child. The findings are published in the British Journal of Obstetrics and Gynaecology
Dr Hollowell comments: 'This finding does highlight a possible anomaly in the guidance given to women on where to give birth.'
First-time mothers with low-risk pregnancies can plan to have their child in hospital, at home, in a free-standing midwife-led unit or an alongside midwifery unit connected to a hospital - according to what is available where they are.
However, current guidelines suggest all women with a body mass index (BMI) over 35 should be advised to have the birth in an obstetric unit to reduce risk.
Guidelines on assessing risks for women in planning births take no account of whether a woman is having their first child or has already had children.
'We think that the guidelines might need to be reviewed to take account of this more nuanced understanding of the effect of obesity,' says Dr Hollowell. 'We think it's worth considering whether it might be safe to allow some healthy obese women having a second baby the option of a birth in an alongside midwifery unit where obstetric and neonatal care is available on site if needed.'
The researchers investigated the influence of BMI on the chances of healthy women having any type of complication during labour and childbirth that would need hospital care - either in an obstetric unit for the mother or a neonatal unit for the baby.
This included a range of medical interventions and complications that varied in urgency and severity, ranging from being given drugs to help with labour progressing slowly, through to having a caesarean section or assisted delivery by forceps or ventouse, significant loss of blood resulting in a blood transfusion, and the baby being admitted to a neonatal unit.
The researchers included data for 17,230 women who'd had straightforward pregnancies and had no medical conditions that might influence risks at labour and birth.
Around half of obese women planning a normal birth after a straightforward pregnancy have no medical or obstetric risk factors such as diabetes or high blood pressure, so this is an important group to study.
The risks of complications during childbirth did go up with increasing body mass index among these healthy women. But the researchers found the increase was modest. Overweight, obese or very obese women showed a relative increase in risk of 6-12% compared to women with a normal BMI.
Most of the additional risk of needing treatment was accounted for by giving drugs for slow labour among those with a high body mass index, though risks of serious outcomes for mother and baby were also increased.
However, whether a woman has had a previous baby plays a larger role in influencing the chances of needing hospital care. 53% of women of normal weight having their first baby had an intervention or a complication at birth. The figure was 21% for very obese but otherwise healthy women having a second or subsequent baby.