Victoria Beckham was back in shape shortly after giving birth to her boys
Trying to copy celebrities who are back in skinny jeans weeks after giving birth is unrealistic and dangerous, new mothers are being warned.
It can take women six months or even a year, not weeks, to shed the weight they put on in pregnancy, say experts.
Crash diets and intensive exercise programmes are not only likely to fail but can harm mother and baby.
The advice comes from Germany's Institute for Quality and Efficiency in Health Care.
According to the Institute, women are under too much pressure to lose weight quickly after giving birth, when many need some of that extra weight to provide nourishment for the baby.
And celebrities who snap back to their normal weight within weeks of having a baby are not a true reflection of real life.
Stars like Nicole Kidman and Victoria Beckham were back in their skinny jeans weeks after giving birth, while model Heidi Klum was on the catwalk in her underwear weeks after having her baby.
The Institute's director Professor Peter Sawicki said: "Often, the extra effort women have to make to look after a new baby and breastfeed after giving birth means the kilos just melt away without effort.
"But for about half of all women, the weight will not go away as quickly."
Those new mothers still in their maternity wear weeks later should not rush to do punishing exercise classes, says the guidance, but should instead follow a sensible diet and build up slowly to more strenuous exercise.
"Even though many magazines have 'get your bikini body back quickly' diets on their covers, promising women they can achieve their ideal weight in time for summer, it is not getting quick results that counts the most.
"This is particularly true after pregnancy. It is normal for it to take three to six months for women to lose the weight they gained in pregnancy," it says.
Professor Sawicki said women could avoid weight problems after birth by eating sensibly during pregnancy.
"It is not a good idea to 'eat for two' in pregnancy and forget about your weight until after the baby is born if you are at all overweight - or prone to overweight - already."
Louise Silverton of the Royal College of Midwives agreed with the advice.
"We would discourage a rush to lose weight after pregnancy, it places undue strain on the mother's body when she needs her strength to look after her baby, and the evidence suggests that slow and sustained weight loss, if needed, is the sensible option.
"Also, following the birth and to breastfeed well, women need a good diet with adequate energy intake, so restricting the amount of food they eat simply to lose weight should be avoided."
UK guidance on weight management for mothers after childbirth is due to be published by the National Institute for Health and Clinical Excellence next year.