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Midwives are calling for official guidance on how much weight is healthy for women to put on during pregnancy.

One in five women in the UK are obese when they start pregnancy, yet there is no national target for what constitutes normal pregnancy weight gain.

Regular weigh-ins fell out of favour in the 1990s when it was suggested that they cause pregnant women unnecessary anxiety for little or no clinical gain.

Officials say they are considering whether to reintroduce them.

The National Institute for Health and Care Excellence (NICE) is reviewing its weight advice for pregnancy.

Its say weight and height should be measured at the first appointment, but not repeatedly during pregnancy as a matter of routine.

do recommend keeping track of weight gain, and set clear targets:

NICE says it is considering whether to adopt the same recommendations in its new advice, which is still in the early phases of planning - meaning it is unlikely to be published before 2019.

Its current guidelines advise against dieting when pregnant, but do say pregnant women should avoid "eating for two". Energy needs do not change in the first six months of pregnancy and increase only slightly in the last three months, when mothers-to-be need around 200 extra calories per day.

Mandy Forrester from the Royal College of Midwives (RCM) said some UK midwives were already using the US weight guidelines, but that others did not have access to weighing scales.

"There is a clear need for midwives to have the tools, guidance and training they need so they can offer women the best possible support and care. This is especially pressing because of the potentially serious complications that can arise in pregnancy as a result of women being overweight or obese."

Women who are obese (with a BMI over 30) when they become pregnant face an increased risk of complications such as gestational diabetes, miscarriage and pre-eclampsia.

Being over or underweight during pregnancy might also have a negative impact on the baby, according to research.

The comments from the RCM come as a new study published in the journal says either too much or too little weight gain during pregnancy is linked with adverse outcomes in children aged seven years.

The Hong Kong study looked at 905 mothers and their children, and found health problems such as high blood pressure and poorer blood sugar control.