A quarter of pregnant women seen by the Mid Yorkshire Hospitals NHS Trust are obese at the time of their first screening.
Health professionals have warned that women could be putting both their own and their baby's health at risk by not losing weight before getting pregnant.
According to data from NHS Digital, around 7,615 women were weighed at their first antenatal booking appointment with the trust in the 12 months to April 2018.
Of these, 25% were found to have a Body Mass Index (BMI) of 30 or above, placing them in the obese range.
A further 28% were deemed overweight, with a BMI of 25 or over.
When the number of underweight women is taken into account, the figures mean that less than half of all expectant mothers seen by the trust were considered to be at a healthy weight at this stage of their pregnancy, which is typically between the ten and 12 week mark.
According to both the NHS and the Royal College of Midwives, obese women are more at risk of a range of complications during pregnancy.
These include a higher chance of miscarriage, high birth weight, gestational diabetes, premature birth, thrombosis, pre-eclampsia and still births.
They may also suffer complications during childbirth, with the RCM warning that overweight and obese women are at a greater risk of having to have a caesarean section or experiencing haemorrhages or shoulder dystocia - where the baby gets stuck during delivery.
A recent US study published in the medical journal BMJ also linked a number of lifestyle factors in mothers, including a high BMI, with an increased risk of their children suffering from obesity during childhood.
Across England, almost half of women were heavier than advised at their booking appointment over the same 12 month period.
Just over 21% were obese, while more than 27% were overweight.
Tam Fry, spokesman for the National Obesity Forum, described the numbers as "saddening".
He added: "Getting into shape before a conception and ensuring that you are eating healthily has been a long advocated message but too few heed it.
"A woman unhealthily overweight at booking in can not only be a health risk for her foetus but also for herself.
"The increasing number of babies being born already obese is also very concerning."
While the NHS advises overweight and obese women to lose weight prior to becoming pregnant, they do not recommend dieting once they have become pregnant.
Clare Livingstone, Professional Policy Advisor at the RCM, said: “While most overweight and obese women will have a straightforward pregnancy and birth, the risk of complication is increased to both mother and baby.
"For those presenting with a BMI of over 30, the National Institute for Health and Care Excellence (NICE) recommend dietary advice, moderate exercise and offer of referral to an appropriately trained professional, for example a dietician.
“Currently there is no consistent UK guidance on safe weight gain in pregnancy and the RCM, together with the Royal College of Obstetricians and Gynaecologists, is calling for evidence to develop guidelines for health professionals.”