Fetal development in pregnancy week 37:fetus in ninth month It’s the calm before the storm. Changes in your baby's weight have leveled off with only a few ounces of fat added this week. At this point your baby should weigh in at around 7 lbs and 20 inches (with boys somewhat heavier and longer than girls). Happily, as far as internal organs go, they are now developed enough to function in the outside world although the oh-so-important immune system is still developing and will continue to do so after birth. With a large boost of antibodies provided by breast milk when nursing begins. Fighting infection and staying healthy should be well within their physical capacity when your little fighter is born.