There are prostaglandins in semen which help to ripen the cervix and orgasming helps start contractions, but it won't start labour if your body isn't pretty well ready to go anyway. If you really cannot fathom DTD at full term then don't do it.

FWIW, I DTD the night that I went into labour with both DS2 and DD1, but it was also about 18 hours after acupuncture each time so who knows. I figured it was worth a shot.