In an original study, researchers in Malaysia found that women who reported having sexual intercourse during the last month of pregnancy were more likely to go into labor before 41 weeks gestation and to avoid labor induction than women who did not have intercourse.  

Of course, this finding is not surprising to midwives.  Women have noticed, probably since ancient times, that intercourse late in pregnancy causes the uterus to cramp or contract, and that a pokey latent-phase labor can sometimes be jump-started with sex.  Sexual intercourse produces prostaglandins from the male and releases oxytocin in the female, so in a way, sex is  a method of labor induction.  It’s a lot more natural and pleasurable
though, not to mention cheaper, easier, and less risky for the mom and the baby, than a regimen of misoprostol in the vagina followed by a synthetic pitocin intravenous drip.  Definitely worth a try!   

Article Citation: Effect
of Coitus at Term on Length of Gestation, Induction of Labor, and Mode of
Delivery; Peng Chiong Tan, Anggeriana Andi, Noor Azmi, and M. N. Noraihan; Obstetrics
& Gynecology 2006;108:134-140.