Post-natal OCD may be a previously unrecognised mental problem faced by many women after giving birth, new research suggests.
Experts estimate that about 11% of women who have recently given birth experience symptoms of obsessive compulsive disorder, compared with 2% to 3% of the general population.
The incidence rate is almost the same as that for post-natal depression, which affects around one in 10 new mothers.
Scientists suspect the conditions may be two sides of the same psychological coin.
Symptoms are generally focused on the baby and include fears about accidental injury, dirt or germs and obsessive checking for mistakes.
In about half the cases investigated, OCD behaviour began to improve after about six months. However, some women only begin to display symptoms this long after delivery and the risk remains for up to a year after giving birth.
"It may be that certain kinds of obsessions or compulsions are adaptive and appropriate for a new parent, for example those about cleanliness and hygiene," said lead researcher Dr Dana Gossett, from Northwestern University in Illinois, US.
"But when it interferes with normal day-to-day functioning and appropriate care for the baby and parent, it becomes maladaptive and pathologic."
OCD is a mental condition characterised by obsessive and frightening thoughts, and an irresistible urge to dispel them.
This can result in repetitive actions, such as constantly washing hands or mental rituals that include counting or avoiding "unlucky" colours. In severe cases the disorder can have a devastating effect on work and social life.
OCD may be triggered by stress, which could explain its association with pregnancy and childbirth, say the scientists.
Manifestations of post-natal OCD include washing and re-washing bottles, and constantly checking that a baby is still breathing or that its cot is secure.
Some women report intrusive fears that they might harm their baby.
The researchers recruited 461 women in hospital to have a baby and screened them for anxiety, depression and OCD.
Tests were carried out two weeks after giving birth and six months after the women had gone home.
The results, published in The Journal of Reproductive Medicine, showed an OCD rate among the women of 11% - around five times the risk for the general population.
About half the women reported an improvement in their symptoms by six months. But some women who had not experienced OCD symptoms at two weeks only then began to suffer them.
About 70% of women who screened positive for OCD were also found to be suffering from depression.
"There is some debate as to whether post-partum (post-natal) depression is simply a major depressive episode that happens after birth, or its own disease with its own features," said co-author Dr Emily Miller, also from Northwestern University.
"Our study supports the idea that it may be its own disease with more of the anxiety and obsessive-compulsive symptoms than would be typical for a major depressive episode."