What Happens During the First Trimester of Pregnancy?

During the first trimester of pregnancy, many changes take place in a pregnant woman’s body, often caused by surges or pregnancy hormones such as estrogen and progesterone. These hormonal surges are generally considered responsible for a variety of pregnancy symptoms and early discomforts a woman may feel. This period also brings many changes and rapid growth in the developing baby.

Many pregnant women don’t know the exact day that they ovulated and conceived. Due to this, the first trimester of pregnancy, which is the first 12 weeks, is usually counted as starting on the day of a woman’s last menstrual period. This may actually be two or more weeks before the egg was fertilized. In general, there are no specific pregnancy-related changes taking place in the body during that pre-fertilization time.

Once the egg is fertilized, it prompts a woman’s body to begin producing higher than normal amounts of hormones, including estrogen and progesterone. These hormones help prepare the body to support the growth of a baby. For example, they play roles in helping to keep the uterine lining thick to protect the developing embryo, and in the development of the placenta, which is the embryo’s key source of oxygen and nutrients.

Other changes in a woman's body caused by pregnancy hormones during the first trimester often include growth of the uterus and breasts, increased blood flow, and slowing of the digestive system. These changes can result in many of the common discomforts that are often felt in the first trimester of pregnancy, such as tender breasts as the milk ducts begin to grow, frequent urination as the growing uterus begins to put pressure on the bladder, and constipation as the digestive system slows to allow more nutrients to be absorbed. One hormone in particular, called human chorionic gonadotropin (hCG), is generally considered responsible for perhaps the most well-known discomfort: morning sickness. These nauseous feelings caused by hCG may actually happen at any time of the day.

The first trimester of pregnancy also brings many changes in the developing baby. Over the first few weeks of pregnancy, the fertilized develops into a tiny ball of stem cells and implants itself in the lining of the uterus. Stem cells are special cells that can develop into any other kind of human cell. Around the fifth week of pregnancy, these special cells begin to differentiate and form familiar human organ systems. At this point, the fertilized egg is now called as an embryo.

The first organ systems that normally develop during the embryonic stage are the nervous system, including the brain and spinal cord, and the circulatory system, including the heart and blood vessels. These generally begin developing during the fifth week of the first trimester of pregnancy. In weeks six through eight, the embryo begins to develop a head, face, eyes, arms and legs. Around week nine, fingers, toes, muscles and bones usually begin to develop. At about 10 weeks, the genitals usually begin to develop.

In general, the last two weeks of the first trimester of pregnancy mark the start of the fetal stage. During this period, the fetus begins to grow rapidly. At the end of week 10, most fetuses are only about 1 inch (2.54 cm) long, but by the end of the first trimester of pregnancy, they have often already tripled in size to about 3 inches (7.62 cm). This rapid growth generally continues on into the second and third trimesters of pregnancy until the baby is born.