Pregnancy is divided up into three stages, which are called trimesters. Pregnancy is counted from week one, before fertilization even takes place, until the end of the third trimester at week 40. Each trimester has its own unique milestones, as the baby grows from a small ball of cells into a full-term infant.
First Trimester
The first stage of pregnancy is the first trimester, which lasts from week one (the start of the menstrual period) to week 13. During this stage, the baby looks a bit less like a baby and more like a small creature with a large head. In fact, the head is almost half of the baby's entire length at this point. Until week 10, the baby is called an embryo. During this time, the organs, brain and heart begin to form. In addition, the embryo sprouts tiny buds that will become its arms and legs. As early as week six, its heartbeat may be visible on an ultrasound, along with the umbilical cord. Around week 10, the baby is considered a fetus. During the remainder of the first trimester, the fetus develops eyelids, teeth and genitals, and takes up the entire uterus. According to the US Department of Health and Human Services (HHS), the baby is about 3 inches long and weighs an ounce by week 12. Despite its tiny size, the baby can already make a fist.
Second Trimester
The second trimester lasts from week 14 to 28. During this stage, the baby's organs and brain continue to grow. The second trimester also marks the formation of the baby's skin, which is transparent for now. The baby's skin is also covered in small, fine hairs called lanugo, and a protective wax-like substance called vernix. There are two milestone events during this stage. The first is fetal movements. Though they actually began during the first trimester, fetal movement may be felt by the mother starting as early as week 16. The second is the ability of an ultrasound to determine the baby's sex. By the end of this trimester, the American Pregnancy Association reports that the baby is a little over a foot long, and weighs just over 2 lbs. If born prematurely at this point, the baby has a chance of survival outside the uterus. According to the Merck Online Manuals Medical Library, this can be as early as the 24th week.
Third Trimester
The third and final trimester lasts from week 29 until the baby is born, which usually occurs sometime between 38 and 42 weeks. During this stage, the baby can open and close its eyes, and may be responsive to bright light. He begins to practice breathing, though his lungs will not mature until around week 37, when the baby is considered to be full-term. The skin becomes opaque and loses its lanugo as the baby continues to develop body fat. By birth, most babies weigh 6 to 10 lbs., and are around 19 to 21 inches long.
