The after-effects of birth play a part. It takes time to recover from stitches, episiotomies, painful breasts and even a "normal" delivery. Penetration can cause pain weeks, even months, after labour. Also, if the woman is breast-feeding, reduced levels of the hormone oestrogen can cause dryness in the vagina, so even if she does feel like having sex, it may be uncomfortable.
Then there is exhaustion. The totally spent feeling that can follow labour and the dragging tiredness of the early months can mean the only thing new mothers crave is mad, passionate sleep. Sex is a loss of valuable sleeping time.
However, post-natal celibacy goes much deeper than the purely physical. Sex is part of something much wider. It is part of adjusting to the different roles and the complex psychological changes that women go through when they become mothers.
"The impact of having a child is much greater on a woman than on a man," said Penny Mansfield, of One Plus One, the marriage and partnership research organisation. "A woman's sense of identity is totally changed when she becomes a mother and that can be quite destabilising."
The birth itself can leave some women feeling violated and exposed. The medical intrusions - internal examinations, being stitched up, the instructions to "open your legs", "show me your tummy", can make a woman feel her body has become public property. Even after the birth, in a subtle way, her "boundaries" are being breached by bleeding, leaking milk and the very physical demands of a new baby. It can make her feel that her body is no longer her own. By the end of the day she is all kissed out and sex seems yet another demand.
Another contributory factor can be what Joan Raphael-Leff, psychoanalyst and expert in pregnancy and motherhood, calls "the mother/ sexual woman divide". "There is the unconscious residue from childhood that mothers are not sexual," she said. "They spend their time looking after others. That's what being a mother is all about." That makes a sexual mother a "bad" mother, so new mothers can feel very inhibited about sex. They are put off by their baby's presence in a way men are not. Some can't even make love with nappies and toys, let alone the baby in the same room. Sexual excitement can seem a potential threat to the child: if a mother gets wrapped up in her own pleasure, she will not hear her baby cry. She will be putting her needs first.
Also, because the sexual ideal is slim and trim, some new mothers with their flabby tummies, stretch marks and droopy breasts feel unattractive. They don't fancy themselves anymore, so can't understand how their partners might.
"Women may also not have sex as a protest," said Penny Mansfield, "one of the ways women express their anger in the post-natal phase is by withdrawing sex." Some mothers, especially those in dual-income couples - may either subconsciously or consciously feel resentful because they feel they are doing everything: having the children, earning the money, running the house and social life. Sex becomes routine and associated with all those other grown-up things like paying the mortgage and having a job.
In fact, studies have shown that the quantity of love-making changes very little as couples get older. But, what does change, is the quality. "In the early years of a relationship there is a very heightened link between physical attraction and love. As couples get older, love associated with different kinds of pleasure: safety, comfort, trust," said Penny Mansfield. "So sex changes. It's inevitable. But there is no need to think that change means that everything has ended and will never be good again." The important thing, says Mansfield, is not to panic. "Having a baby is a huge transition in life, especially for women, and does need working through." There is sex after childbirth - often a more meaningful, intense sort of sex, but sometimes only once the mother has adjusted to her role, the mother rested, after her body is her own again.
`Is There Sex after Childbirth?' by Juliet Rix, Thorsons, pounds 7.99.