It is not advisable to have sex too soon after childbirth because of the (very slight) risk of embolism.
I can't link the 1998 original research in the Postgraduate Medical Journal as it is not currently available from the archives, but I have found a report of the research from the Los Angeles Times.
Extract:
"For most couples, the idea of having sex one week after having a child might be the furthest thought from their minds. But just in case, be warned that British doctors have found that sex soon after birth can be dangerous and, in very rare cases, even fatal.
Intercourse too soon after childbirth can cause a deadly air embolism, or bubble, in a blood vessel, a risk that remains for six weeks after birth while the mother's womb is returning to normal and the spot where the placenta was attached is healing, according to a report in the October Postgraduate Medical Journal.
"It can be fatal. It is very, very rare," according to Dr. Philip Batman, a pathologist at the Bradford Hospitals NHS Trust in northern England. "It does happen, and obstetricians should be aware of it." Batman and his colleagues described the cases of two young mothers from northern England who died within a two-year period as a result of making love shortly after having a child. A 22-year-old woman suddenly collapsed and died while having sex with her husband eight days after delivering her third child. The second woman, a 29-year-old with four children, suffered a similar death five days after giving birth. Both deaths were caused by air embolisms."