Week 17: Onion
Baby's skeleton is hardening, changing from rubbery cartilage to bone, and fat is finally accumulating around it. The umbilical cord is getting thicker and stronger, and those little fingers and toes are now topped by one-of-a-kind prints.
Week 18: Sweet Potato
Your fetus has become amazingly mobile (at least compared to you), passing the hours yawning, hiccuping, rolling, twisting, kicking, punching, sucking and swallowing. And, baby's finally big enough that you'll be able to feel those movements soon.
Week 19: Mango
Vernix caseosa, a greasy white substance made of lanugo, oil and dead skin cells (yum) now coats baby's skin, shielding it from the amniotic fluid. (Picture yourself after a nine-month bath, and the need for protection makes sense.) You might actually get to see the vernix at birth, especially if baby is premature.
Source: the bump