The first trimester is a period of rapid growth in which the fetus’ major external features begin to take shape. Each week of the first trimester involves some significant new milestone.

The first trimester typically unfolds as follows:

• Week one: Conception typically occurs about two weeks after a woman’s period begins, with the due date following 40 weeks after the beginning of the woman’s last period. However, fertilization may occur at other times during the menstrual cycle depending on when the individual woman ovulated.
  
  
• Week two. A zygote is formed when the sperm and the egg come together in a woman’s fallopian tube. A zygote contains 23 chromosomes from each partner, for a total of 46. Chromosomes are the genetic information – also known as DNA – that determines a person’s traits (e.g., gender, eye and hair color, height). In most cases, just one zygote is formed, but multiple zygotes may develop. The zygote moves down the fallopian tube and toward the uterus. As it does so, it divides into a ball of cells.
  
  
• Week three: Further cell division creates an inner cluster of cells that has an outer shell. This combination is known as a blastocyst. The inner group of cells eventually will become an embryo while the outer group of cells becomes the membranes that nourish and protect the developing embryo. The blastocyst continues toward the uterus. Once the blastocyst reaches the uterus, it implants into the lining of the uterus known as the endometrium. The blastocyst receives nourishment from the mother’s bloodstream. The placenta, amniotic sac and umbilical cord begin to form. These structures provide nourishment and oxygen to the embryo and carry away waste.
  
  
• Week four: The embryo’s organs begin to form at this point, known as the embryonic period. The embryo becomes three layers. The neural tube that eventually will become the central nervous system (CNS) forms in the top layer of the embryo (ectoderm). It will house the brain, spinal cord and nerves, and backbone. A neural crest also forms, which will become the peripheral nervous system. The heart, circulatory system, bones, muscles, kidneys and much of the reproductive system begin to form in the middle layer of the embryo (mesoderm). The lungs, intestines and bladder develop in the inner layer of the embryo (endoderm).
  
  
• Week five: The heart and circulatory system continue to take shape, and the embryo’s heart begins to beat as the blood vessels come together. At this point, only ultrasound can detect the heartbeat. Circulation begins. The second month of development is a crucial time, as the embryo is especially vulnerable to damage from drugs, viruses or environmental factors (such as pesticides). The embryo now is 1/17 of an inch (.15 centimeters) long and more closely resembles a tadpole than a human at this stage of development.
  
  
• Week six: The embryo’s heart begins to beat with a regular rhythm, and facial features begin to take shape. These include an opening for the mouth and inner ear passages. The digestive and respiratory systems begin to form, and the beginnings of the embryo’s connective tissue, ribs and muscles emerge. Arms and legs appear as small buds. By this point, the embryo is about 1/8 inch (0.32 centimeters) long.
  
  
• Week seven: The umbilical cord has formed by this week. Though the embryo’s skull remains transparent, the passageway needed to circulate spinal fluid in the brain is already intact. The arms and face begin to be more clearly defined. By the end of the seventh week, virtually all essential organs have begun formation.
  
  
• Week eight: Webbed fingers and toes are present, as are clearly defined wrists, elbows, ankles, ears, upper lips and tip of the nose. The eyelids begin to form, and the embryo’s heart rate is about 150 beats per minute. By the end of the second month, all of the major body organs and systems have begun to develop.
  
  
• Week nine: Although expert opinions vary as to the exact timing, the ninth week can be seen as the approximate time when the embryo becomes a fetus. The fetus begins to take on more of a human appearance as the tail at the bottom of the spinal cord begins to disappear. Organs that have developed include the anus, bile ducts, gallbladder and pancreas. Nipples, hair follicles and internal reproductive organs (testes and ovaries) begin to form. The fetus sometimes begins moving at this point, although the mother cannot feel these movements.
  
  
• Week 10: The fetus’ tail has disappeared, as has the webbing between the fingers and toes. The skeleton begins to form at this point, and the brain begins to form neurons at the rate of 250,000 per minute. The eyelids lose their transparency, outer ears begin to assume their final form, and tooth buds first appear. Male fetuses begin to produce the hormone testosterone from their testes, which are still located in the abdomen at this stage.
  
  
• Week 11: Blood vessels in the placenta grow larger and begin to multiply to provide the fetus with the nutrients it needs. The ears move up toward the side of the head, and external genitalia (penis and labia majora) can be distinguished in an ultrasound at this point.
  
  
• Week 12: Fingernails and toenails develop, while the chin and nose become more defined. A fetus averages nearly 3 inches (7.62 centimeters) long at this point and weighs 4/5 ounce.