Men accompany the mother of their child on a nine-month hormonal roller coaster, it has been discovered.

Canadian researchers have shown that expectant fathers' hormones fluctuate up and down, mimicking their partners' levels.

During pregnancy, levels of several hormones rise in an expectant mother; prolactin which triggers lactation, cortisol which is related to a mother-baby bonding and the main female sex hormone, oestradiol. Immediately after the birth, the levels crash.

Anne Storey of Memorial University, Newfoundland, and her colleagues wanted to find out if the same was true for men.

Drastic changes

New Scientist reports that they recruited 34 couples from an antenatal class and took blood samples at different times during and after the pregnancies.

Pregnant man: A 1970s Health Education Council advertisement

The researchers found that in fathers, levels of cortisol, prolactin and testosterone changed significantly during their partners' pregnancies.

"The differences for mums were much more drastic, but the patterns were similar," says Dr Storey.

Testosterone dropped by 33% just after the baby's birth, and the lower levels of testosterone were associated with men becoming more parental, the researchers found.

Dr Storey speculates that a combination of behaviour and pheromones from a pregnant woman somehow prompts the father to prepare for the birth of his child: "There's something about the couple being together that sets the stage."

Crying on cue

The team also asked fathers about changes that might signal a "sympathetic pregnancy", such as fatigue, change of appetite and weight gain. Fathers who reported these symptoms also had higher prolactin levels and a steeper drop in testosterone than those who did not.

Finally, the researchers studied short-term changes. After giving a blood sample, volunteers listened to a six-minute tape of a newborn baby's cries and watched a video about the difficulties of learning to breastfeed. Thirty minutes later, their blood was sampled again.

The researchers found that both the men and the women experienced pronounced hormonal changes after exposure to the baby cues, with cortisol levels plummeting.