What Does a Baby Develop in the First Trimester

Stages of Fetal Development - First Trimester

One - 13 Weeks

FERTILIZATION (2 weeks after the first day of the last normal menstrual period) Biologically speaking, fertilization (or conception) is the beginning of human development. Fertilization normally occurs within several hours of ovulation (some authors report up to 24 hours) when a man's sperm combines with a woman's egg inside a woman's uterine tube.

WEEK 2 (4 weeks after the first day of the last normal menstrual period) Implantation begins the first week and the embryo continues to grow. The embryo is about¹/₁₀₀ of an inch long at this time.

WEEK 4 (6 weeks after the first day of the last normal menstrual period) The embryo is about ¹/₆-inch long and has developed a head and a trunk. Structures that will become arms and legs, called limb buds, begin to appear. The brain develops into five areas and some cranial nerves are visible. The eyes and ear begin to form. Tissue forms that develops into the vertebra and some other bones. The heart continues to develop and now beats at a regular rhythm. Rudimentary blood moves through the main vessels.

WEEK 6 (8 weeks after the first day of the last normal menstrual period) The embryo is about ¹/_2-inch and has a four-chambered heart and nostrils. Electrical activity begins in the developing brain and nervous system. The brain continues to form. The lungs begin to form. Fingers and toes begin to form, and arms and legs have grown longer. Feet and hands can be distinguished and now have fingers and toes (digits), which may still be webbed. The shell-shaped parts of the baby's ears are forming, and the baby's eyes are visible. The upper lip and nose have formed. The trunk of the baby's body is beginning to straighten.

WEEK 8 (10 weeks after the first day of the last normal menstrual period) The fetus, until now called an embryo, is about 1¹/₄-inches long, with the head making up about half this size. The beginnings of all key body parts are present, although they are not completely positioned in their final locations. Eyes, ears, arms and legs are identifiable. The neck begins to develop, and the baby's eyelids begin to close to protect his or her developing eyes.

WEEK 10 (12 weeks after the first day of the last normal menstrual period) The fetus is about 2¹/₂-inches from head to rump and weighs about ¹/₂ ounce. Fingers and toes are distinct and have nails. The fetus begins small, random movements, too slight to be felt. The fetus's heartbeat can be detected electronically. The baby's face now has a human profile.

WEEK 12 (14 weeks after the first day of the last normal menstrual period) The fetus is about 3¹/₂-inches from head to rump and weighs about 1½ ounces. The fetus can swallow, the kidneys make urine, and blood begins to form in the bone marrow. For females, ovarian follicles begin forming. For males, the prostate appears. A doctor may be able to identify the sex through special tests.