Weeks Six Through 13
About a month and a half after conception, you will almost certainly notice that you need to urinate more frequently. If you have them, your morning sickness and fatigue may continue through this time. You may be experiencing some constipation and perhaps heartburn. If you have not yet seen changes in your breasts, you may notice these now. You may also experience mood changes and irritability, similar to premenstrual mood swings.
As early as the sixth week of your pregnancy, unbelievable as it seems, the baby's heart, although not fully formed, begins to beat. Buds form where arms and legs will soon appear, the head is beginning to form and mild indentations mark the location of eyes. By eight weeks, the embryo is not quite human looking with a disproportionately large head, discernible toes and fingers, and tightly closed eyes covered with fine skin. The ears, ribs, arms, legs and spine are already beginning to harden, although the muscles are not working yet. The genitals are visible but incomplete at this point, and a lower jaw is taking shape. The fetus swallows and, even this early, expels urine through its bladder into the amniotic fluid. All these structures are still quite small and many visible just under the microscope. If you have reason to have an ultrasound this early in pregnancy (because you are bleeding, for example) don't expect to see much more than the flicker of a heartbeat. Ultrasound cannot, for example, determine fetal sex until much later in pregnancy.
You will notice new changes in your body by the twelfth week. By week 12, most women report that morning sickness is improving. The nipples and areolae of your breasts have darkened. The uterus can now be felt through the abdomen as it begins to elevate above the pelvis.
Between week 11 and 13, your doctor may talk to you about genetic screening. These tests — blood tests and ultrasounds — can help women learn more about the chances their pregnancy is affected by certain birth defects or inherited (genetic) disorders. Blood tests measure the levels of certain substances present in a woman's blood during pregnancy. Abnormal levels can indicate the possibility of a neural tube defect (a problem with the skull or spinal cord) or the possibility of chromosomal problems (such as Down syndrome). Ultrasounds look for abnormalities in the fetal anatomy, such as a heart malformation. Sometimes the ultrasound shows something abnormal, but the baby turns out to be normal. Your doctor might recommend a diagnostic test, such as amniocenteses, if the ultrasound is abnormal.
By week 13, the embryo is already moving but, because it is small in relation to the uterus, you cannot yet feel the fetus moving. Its spine and internal organs have begun to form. The fetus has grown to two to three inches in length and weighs about an ounce. The head and body have grown and teeth buds are evident in the lower jaw, although, again, most of these changes are too small to be seen by even the best ultrasound equipment. Incredibly, sucking begins at this early stage, and the baby routinely swallows amniotic fluid. The muscles begin to grow and the brain begins to coordinate their movement. The umbilical cord passes from the placenta to the fetus and carries waste from the fetus back to your circulatory system. At the end of the first trimester, the pregnancy is now referred to as a fetus rather than as an embryo.