Mothers-to-be think their own mothers know better than the medical profession when it comes to health advice, researchers say.
A University of London team talked to women who gave birth in the 1970s, 1980s and the 2000s.
Modern women were more likely to take a mixture of advice - but were still more likely to follow family wisdom.
One baby charity said family tips were useful, but medical advice should be sought if mothers-to-be had worries.
The researchers talked about pregnancy and childbirth advice to seven women who gave birth in the 1970s and 12 of their daughters who had babies in the 2000s.
They then also analysed interviews on the same topic which had been carried out with 24 women in the 1980s.
The 1970s women were most likely to take advice from family members.
But researchers found that women who had babies between 2000 and 2010 had to evaluate a wide range of information from doctors, midwives, books, magazines and, latterly, the internet - as well as that from their families.
In these women, it tended to be family advice that won out - particularly if a mother-to-be was dealing with a specific symptom.
One woman, Hetty, from the 2000s generation, said she had tried to stop drinking tea because she had read on the internet that caffeine could cause miscarriages in the first few weeks of pregnancy.
But she then added she had taken her grandmother's advice that tea could help relieve morning sickness.
"She just used to stay in bed and have a cup of tea. And that did help actually."
'Strike a balance'
Professor Paula Nicolson from Royal Holloway, University of London, who led the study, said: "When it comes to the crunch - if women feel sick for example - they will take their mother's or their grandmother's advice.
"They wouldn't necessarily recognise how important it was to them, but it would override the science."
She added: "Taking all the guidelines too seriously leads to anxieties. Lack of self-confidence also can lead to worry about 'doing the wrong thing' which is potentially more harmful than taking the odd glass of wine or eating soft cheese."
Jane Brewin, chief executive of baby charity Tommy's, said women had to "strike a balance" about what advice they took.
"It's only natural to want to talk about the significant changes that happen to a woman's body and how she feels; mums and close friends often have first-hand experience and tips that are helpful.
"However we always stress that if any mum-to-be is worried about anything during their pregnancy they should seek medical advice without delay."