Here's the 18-week update from BabyCenter.com:

Head to bottom, your baby is approximately 5 1/2 inches long (about the length of a bell pepper) and weighs almost 7 ounces. He or she's busy flexing her arms and legs — movements that you'll likely start noticing more and more. Blood vessels are visible through his/her thin skin and ears are now in position and stand out from the head. Myelin (a protective covering) is beginning to form around the nerves, a process that will continue for a year after he/she's born. If you're having a girl, her uterus and Fallopian tubes are formed and in place. If your baby is a boy, his genitals are noticeable, though he may hide them from you during an ultrasound.