36-38 weeks from conception is considered "full term" - the time at which all of the baby's organs are usually fully ready for life outside of the uterus. The weeks of pregnancy, however, are not counted from the time of conception, but rather from 2 weeks before conception (in a woman with a 28-day-cycle, this would be the date of her LMP - last menstrual period). Since an additional two weeks generally pass before a woman discovers that she is pregnant, at that time, she is already 4 weeks pregnant, with her due date being 36 weeks from then.
Up to the end of the 24th week, the fetus is not considered viable (it has less than a 10% chance of survival if born in the 23rd week). By the end of the 24th week, the survival rate is somewhere around 55%, though some babies will have lasting difficulties associated with prematurity, including blindness and retardation. As the pregnancy progresses, the survival rate continually improves, with the rate of complications gradually falling. By the end of 27 weeks, the survival rate nears 90%. Survival, however, is not enough to consider it a 'safe' time to give birth.
A majority of babies born after 32 weeks of pregnancy will not have lasting difficulties. By 37 weeks, most babies will be released from the hospital just like full-term babies, requiring no extra care. This is the time at which I would consider it safe to give birth. The vast majority of babies is born between 38 and 42 weeks - a woman who is still pregnant beyond 40 weeks, will be monitored more closely and induced at the first sign of fetal distress, deterioration of the placenta or pre-eclampsia in the mother. Most medical centers induce birth by the end of 42 weeks.
