Your baby is now probably between 5 and a half and 6 and a half inches long from head to rump and weighs about 9 ounces. She may be halfway home, but at week 20, she has a lot of growing to do. By the time she's actually born, she'll be about 13 times heavier than she is now. (Luckily, your own weight gain will be a bit more modest.)
Her features are starting to take shape. She now has eyebrows, fingernails, and toenails. Her skin is getting thicker, and her entire body is covered in lanugo (fine hairs). At this age, a girl will have about 6 million eggs in her ovaries. She'll only have about a million left by the time she's born.
By now, you've probably felt her moving around. Those aren't just random squirms. She now has the brainpower to move on purpose. She'll kick, reach, and suck her thumb. Her brain is also starting to get a glimmer of all five senses: sight, sound, touch, smell, and taste.
Your doctor might suggest an ultrasound exam at this time. This is one exam both you and your partner can look forward to. You'll get to see your baby, from her head to her spine to her beating heart. If your baby is sitting in a cooperative position, the ultrasound operator might also be able to see whether your baby is a girl or a boy. Of course, ultrasounds aren't just for your enjoyment. This exam will give your doctor a chance to monitor your baby's growth and check for potential signs of trouble. You should know, however, that ultrasound can't always detect birth defects with total accuracy.
Ultrasound exams are considered completely safe for both mother and baby, but doctors still don't prescribe them casually, because the effects of frequent ultrasounds are not known. If there's no reason for concern, you may not get another look at her until she's born.
Campbell, Stuart MD. Watch Me Grow. St. Martin's Griffin.
Curtis, Glade MD and Judith Schuler. Your Pregnancy Week by Week. Da Capo Press.
Nemours Foundation. Pregnancy Calendar. 2010. http://kidshealth.org/
American Pregnancy Association. Ultrasound Updated 03/2006 http://www.americanpregnancy.org/prenataltesting/ultrasound.html
March of Dimes. Ultrasound January 2009 http://www.marchofdimes.com/professionals/14332_1167.asp
Shanahan, M. Kelly, MD. Your Over-35 Week-by-Week Pregnancy Guide. Prima Publishing.