A craving for sweets, early morning sickness and a watermelon-shaped stomach are all said to indicate that a woman will give birth to a baby girl.
But an intriguing study suggests it is possible to predict the sex of a successfully born baby months before it is even conceived.
Scientists in Canada discovered that a woman's blood pressure at around 26 weeks before conception predicts if she will give birth to a boy or a girl.
Higher systolic (maximum) blood pressure signals she will deliver a boy, while lower pressure suggests a girl.
Dr Ravi Retnakaran, endocrinologist at Mount Sinai Hospital in Toronto, said: "It suggests that a woman's blood pressure before pregnancy is a previously unrecognised factor that is associated with her likelihood of delivering a boy or a girl."
In the study, the mean systolic blood pressure for women who had boys was 106mm Hg, compared to 103mm Hg for those who had girls, in the months leading up to conception.
"When a woman becomes pregnant, the sex of a foetus is determined by whether the father's sperm provides an X or Y chromosome and there is no evidence this probability varies," added Dr Retnakaran. "What is believed to vary is the proportion of male or female foetuses lost during pregnancy. This study suggests that either lower blood pressure is indicative of a mother's physiology that is less conducive to survival of a male foetus or that higher blood pressure before pregnancy is less conducive to survival of a female foetus."
For the study, published in the American Journal of Hypertension, more than 1400 newly married Chinese women were recruited, all of whom were trying to become pregnant. Their blood pressure was checked at around 26 weeks before conception and they were followed through pregnancy.
After adjustment for age, education, smoking, Body Mass Index (BMI), waist, cholesterol, triglycerides (fat in the blood) and glucose, mean systolic blood pressure before pregnancy was found to be higher in women who subsequently had a boy than in those who delivered a girl.
Fertility expert Prof Charles Kingsland, of Liverpool Women's Hospital, said: "We have been aware that more male foetuses miscarry than females. There is also some evidence that you are more likely to miscarry a boy when you are compromised either by health or environmental issues. So I suppose, blood pressure changes in these circumstances might affect the conception of different sexes."
Geoffrey Trew, a consultant in reproductive medicine and surgery at the Hammersmith Hospital, cast further doubt on the findings. He said: "I would be very surprised that a BP measurement, which is notoriously variable, could dictate sex 26 weeks before."
The Telegraph, London