Your Baby's Development
Your baby now weighs about 2.8 ounces (80 grams) and measures about 4.6 inches (116 millimeters) in length. Although your baby's first muscle movements were involuntary, this week brings the first voluntary muscle movements. Your baby can hold his or her head erect, and the development of facial muscles allows for a variety of expressions such as squinting and frowning.

More calcium is deposited on your baby's bones as the skeleton continues to develop. In addition, if your baby is female, millions of eggs, or ova, form in her ovaries this week.

Your Body
Around the sixteenth to eighteenth week of pregnancy, your health care provider will likely offer you the maternal blood screening test, also known as a "triple marker" test or a "triple screen," which measures the levels of the alpha-fetoprotein (AFP), a protein produced by the fetus, and the pregnancy hormones hCG and estriol in the mother's blood. The results of the triple marker test can tell moms whether their babies are at risk for (not whether they have) neural tube defects such as spina bifida or chromosomal abnormalities such as Down syndrome. Out of every 1000 women who take the test, 50 will have abnormal results, but only one or two women will actually have babies with a neural tube defect. Talk to your health care provider about the risks and advantages of this test.