The First Trimester

The first trimester of pregnancy is the first 12 weeks after the first day of the last menstrual period. This trimester is the most consuming for many women because everything is so new, so exciting, and so overwhelming all at once. The first trimester is also one of amazing development in the baby. Here is some basic information you should know about these first 12 weeks. View a week-by-week view for fetal development, or click to learn more about pregnancy at 1-4 weeks, 5 weeks, 6 weeks, 7 weeks, 8 weeks, 9 weeks, 10 weeks, 11 weeks, 12 weeks, 13 weeks or 14 weeks. If you're a member, you can customize your dates and follow along with the Pregnancy Calendar, too!

Fetal Development

Fertilization (About 2 weeks after the first day of the last menstrual period): The mother's body releases an egg, which is fertilized by a sperm. The fertilized egg has the 46 chromosomes needed to determine height, hair color, and sex. The egg begins to divide, and begins its trip down the mother's fallopian tube toward the uterus.

Implantation (About 5-7 days after fertilization): The egg attaches to the lining of the uterus during the first trimester of pregnancy. The fertilized egg begins to grow and doubles in size every day. The placenta and umbilical cord begin to form and carry nourishment and oxygen to the baby and carry waste away.

Week 4 (2 weeks after conception): This is about the time that the mother misses her period. The embryo is about 1/100 of an inch long at this time. The baby has a spinal cord, and is dividing into three layers that will become the different organs and systems in the baby.

Week 6 (4 weeks after conception): The embryo is about 1/6 inch long. Limb buds, which will become the legs and arms begin to appear. Blood is being pumped through the fetal circulation, and a heartbeat can be seen on an ultrasound.

Second Month: At the beginning of this month, the baby is about 1/2 inch long and weighs a fraction of an ounce. The baby's development is very rapid during this month. By the end of the second month, all of the baby's major body organs and body systems have begun to develop. At the end of the second month of pregnancy, your baby looks like a tiny human infant. The baby is a little over 1 inch long and still weighs less than 1 ounce.

Third Month: The baby is now officially called a fetus, and will be completely formed by the end of this month. The baby is beginning to move its hands and legs, and opening and closing its mouth, but he or she is still too small for you to feel the movement. Fingers and toes are now distinct, and hair is beginning to form on the head. By the end of this month, the baby will be about 4 inches long and weigh just over an ounce.

Mother's Body

Pregnancy Symptoms: About the time that a woman misses her period, she often also starts to experience the symptoms that are common to pregnancy, including nausea, headaches, dizziness, breast changes, frequent urination, heartburn, aversions, and cravings. These symptoms are caused by hormones, and typically are more severe in the first trimester. One of the most common complaints of the first trimester is morning sickness, which can range from mild to severe, and usually lessens around the end of the first trimester.

Emotional Changes: Emotions are also effected by the raging hormones of the first trimester of pregnancy. Many women feel irritable, have mood swings, and are quick to cry. It is also common, whether the pregnancy is planned or unplanned to have a wide range of feelings about the pregnancy including joy, fear, excitement, and apprehension.