Why do some men experience pregnancy symptoms such as vomiting and nausea when their wives are pregnant?

Sounds funny? This post is not to make fun but to take SCIENCE seriously.
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When pregnancy symptoms such as nausea, weight gain, mood swings and bloating occur in men, the condition is called couvade, or sympathetic pregnancy. Depending on the human culture, couvade can also encompass ritualized behavior by the father during the labor and delivery of his child. Couvade has a long anecdotal history and is named from the French verb couver, which translates as "to hatch" or "to brood." However, this phenomenon has received attention from biologists quite recently.

Society and health professionals all show a lot of interest in a pregnant woman. She is encouraged to talk about any symptoms of her pregnancy, even common ones she is not experiencing. At home, the conversation can range from frustrated incapacitation as a result of her symptoms to boundless joy in anticipation of a child. Thus, it is not surprising that a large number of mental health professionals have considered a range of hypotheses--from jealousy about a man's inability to carry a child to guilt over having caused this transformation in his partner to selfish attention seeking--as the root causes of couvade.

There is no doubt that testosterone concentration is lower in the men in relationships, but it is unclear whether men have a decrease in testosterone after the relationship begins or whether men with lower testosterone are more likely to enter into stable relationships. Similarly, there are hormone changes associated with fatherhood. Prolactin is highest in men in the weeks just before the birth, testosterone is lowest in the days immediately after the birth, estradiol levels increase from before to after the birth, and cortisol peaks during the labor and delivery. Hormone changes in expectant fathers therefore involve the same hormones that are changing in an expectant mother. They are also the same hormones that are implicated in animal models of parental behavior. In addition, it is known that men are not reflecting a "muted" version of the hormonal experiences of their partner: Within couples, day-to-day hormone status is not correlated during pregnancy or after the birth.

Unfortunately, these data remain correlations, and the exact role of hormones in facilitating paternal behavior or causing couvade symptoms in expectant fathers remains unknown. It is certainly tempting to look to hormones for the biological root of couvade symptoms, but caution is needed. Other events also happen around a pregnancy, especially the birth of a first child, and could independently affect hormone concentrations. Changes in sexual activity, shifts in the social priorities of the couple, time off work, or the arrival of a mother-in-law for a potentially stressful extended visit are obvious candidates. Of course, even if the stimuli causing the hormonal changes are not the result of an approaching birth the hormone changes might produce couvade symptoms and/or facilitate a father's social bond with his child. Either way, this kind of research has quietly expanded the horizons for research on hormones in men--testosterone alone is clearly no longer the sum of the man.