(British Medical Journal) -- Women are more likely to encounter complications during labour and delivery when they are having a boy, according to researchers in this week's BMJ.

The study took place in the National Maternity Hospital, Dublin using data from the delivery ward database during 1997 to 2000. All mothers who were pregnant for the first time with a singleton birth and who spontaneously went into labour at term were included.

In the study period 4,070 male and 4,005 female infants were analysed. Male infants were significantly more likely to require oxytocin (a hormone that stimulates contractions), blood sampling, and instrumental vaginal delivery or caesarean section. There was no significant difference between the sexes in other variables studied, such as gestation or requirement for antibiotics during labour.

Male infants have a significantly larger head size than female infants, and this may contribute to the duration of labour and the higher incidence of operative delivery, say the authors. However, this factor would not fully explain the sex difference, as duration of labour alone would not account for the increased incidence of suspected fetal distress in males (as shown by more need for blood sampling in boys).

"What this study does show is that when we say "it must be a boy" as a humorous explanation of complications of labour and delivery we are scientifically more correct than previously supposed," they conclude.