Google to the risk...this BBC article on this very topic is very interesting.

http://news.bbc.co.uk/2/hi/health/253374.stm

Quote:

Infertility doctors have long been aware of "spontaneous conception" - a naturally occurring pregnancy after the birth of a child conceived through in vitro fertilisation.

Now a Japanese study points to who is most likely to benefit from the effect.

It found that overall there was an 18% chance of spontaneous conception after IVF treatment, but that the likelihood rose to 37% among women under the age of 27.