Belly dancing in pregnancy? Isn't that a bit, well, weird? Well, no, actually. Not only is it that rare breed of exercise for which having a bulge is an advantage, but belly dancing was once a symbol of maternity in the Middle East, where it was traditionally performed (by women, for women) as a celebration of fertility, conception and birth.

While modern womenfolk seem unlikely to gather female friends around them to dance while they give birth, belly dancing can strengthen the internal support systems of the body during pregnancy. Forget "the quick dance of Turkish belly dancers in nightclubs with horsetails, and diamonds in their navels", says social anthropologist Sheila Kitzinger. The dance you need is slow and languorous, with gently undulating, rolling moves.

At Jacqueline Chapman's Belly Babies classes in Chislehurst, Kent, most moves start with the basic stance: knees bent, tail tucked in and an imaginary pencil clamped in the vagina. One in four women are affected by stress incontinence after birth, yet exercising the pelvic floor can make a difference. "It's just another bit of you to exercise, like your arms or legs," says Chapman, who champions a regular perineal workout. "If you don't use these muscles, they go slack and flabby."

Belly dancing in pregnancy isn't only about a strong pelvic floor, however. The belly roll (a kind of undulation of the abdominal muscles) can help ease constipation; heart shimmies (a gentle shaking or vibrating of the chest) can alleviate heartburn; the camel roll (which is similar to a belly roll) encourages optimal foetal positioning; and hip circles and figure eights are useful for strengthening the muscles used during birth. Some moves are strictly for birth, such as the tension-releasing full-body shimmy, which involves a vibration that starts at the feet and goes all the way to the top of the head; and the vertical hip shimmy, for the final stage of labour only, which "relaxes the whole pelvic floor, allowing foetal descent", says US-based midwife Cathy Moore.

"Belly dancing exercises all of the muscle groups specifically used for giving birth," Moore adds. Moore, who attends hospital births in Boston and teaches belly dancing, says that the basic posture alone strengthens the leg muscles needed to support an upright birth position - known to reduce pain, shorten the first stage of labour and result in fewer assisted births.

Like any exercise during pregnancy, moderation is recommended. "Finding a class tailored specifically towards pregnancy is important," says Louisa Matthews, who teaches in Leicester and advises clients to wait until the 12-week mark to start.

But anything that lifts the spirits has got to be good for the foetus. We know babies can hear in utero from 20 weeks. So, while they listen to music, let them also enjoy a little soothing rocking and internal massage while you gently shake your thing.

· Jacqueline Chapman bellydancer.org.uk

· Louisa Matthews goddessbellydance.co.uk

· Cathy Moore thegoddessdancing.com

· Active Birth Centre activebirthcentre.com