Not much research has been done to establish the frequency of intercourse after the birth of a child.  The only information that I could find comes from The BabyCenter Sex Survey; What new and Expectant Parents are really doing http://www.babycenter.com In this survey, they asked the question How soon after your baby was born did you get back into the swing of things?

Nearly half of all couples followed their doctor's instructions and waited until the second month to have sex (most doctors recommend waiting four to six weeks). Still, 20 percent had to have each other in the first month — with one couple admitting they waited just one day! Only a fraction of couples waited more than six months to make love again.

48.2% — We had sex again in our second month.
21.2% — We made love within the first month.
18.6% — We had sex for the first time somewhere between months three and four.
2.8% — We had sex for the first time somewhere between months five and six.
2.5% — We waited at least six months to resume lovemaking.
6.8% — Other

Typically, physicians recommend women wait to resume intercourse until after they have been seen for their post-birth check-up around 4-6 weeks after birth.  Doctor's recommendations to avoid intercourse during the post-partum period stem from a desire to minimize the risk of infection from spreading up through the vaginal canal into the uterus, before the uterus has an opportunity to return to it's 'normal' pre-pregnancy state. Nevertheless, it seems as if many people opt to take matters into their owns and have sex sooner than the "doctor recommends".  Interestingly a significant number of women will wait even longer to resume sexual relations despite having a normal exam. Some of the causes for this might stem from

• Exhaustion / fatigue
• Lack of desire
• Pain during intercourse.  While a woman might have a 'normal' exam, sometimes it takes a few extra months for the inside of the vaginal canal to feel normal.  However, given the trauma that the woman just experienced while giving birth, please make an appointment with your obstetrician or midwife to rule out any potential medical complications.