This week, you've reached the end of your second trimester, and so has your baby! Looking back just 13 weeks ago, your baby's weight barely reached an ounce. Now she's about 2 pounds. All of her major body systems are developed and working, although she still needs time for these systems to mature and to put on weight. If your baby were born at this point, she'd have around a 90 percent survival rate.
While your first trimester is truly a time of development, the second builds on those new formations and adds definition. Take your baby-to-be's ears, for example. Around eight weeks into your pregnancy, your baby's ears begin to take shape. They are low on her oversize head. But as she grows, and her body comes into balance with her head, her ears move into their final position so that by week 15 they're not only in the proper place, but nerve endings are working their way between her ear and her brain so that she can hear. While the sounds might still be muffled, especially since the thick vernix caseosa is covering her body, she can begin to distinguish between a dog barking and the sound of your voice. By the end of the second trimester, her hearing is well-developed and she'll even respond in utero to sounds outside the womb.
Her arms and legs are another example of the growth process happening in the second trimester. In the first trimester, her appendages began as tiny buds in the sixth week of pregnancy (four weeks' gestation). In the weeks that followed, her arms and legs became elongated and bones began to form under her translucent skin. But by the second trimester her arms and legs are nearly the size they'll be at birth -- and they're proportionate to the rest of her body. The growth taking place moves inward instead of outward. For example, two weeks into the second trimester, the bones begin to harden in a process called ossification. And by week 22, her bones are beginning to produce fetal red blood cells. At birth, her bones will be able to provide all the red blood cells her body needs.
Your baby is still kicking so much that you might worry if she's suddenly quiet. Don't. How much a baby kicks has more to do with her personality than with her health. If you want to reassure yourself that everything is fine, you can do an average "kick count." Lie on your left side -- usually early evening after you've eaten is the best time -- and check to see how long it takes to feel 10 movements. Do that every day for a week; then average the number of kicks to see what you should expect. Remember too that your baby's movements will naturally change as you enter your ninth month and her bedroom becomes more cramped.
In the third trimester, your baby-to-be enters a phase of weight gain and developmental maturity. Before long, she'll be ready to greet you so that you can look at all her amazing growth, her perfect fingers and toes, for yourself!
Erythropoiesis: The production of red blood cells.
Images courtesy of the American Institute of Ultrasound in Medicine (AIUM.org).