The following is a daily, weekly, and monthly description of what is happening in the uterus and the developing fetus from conception, which is two weeks after the last menstrual period. (The actual pregnancy began two weeks ago, on the first day of the woman's period. During the two weeks before conception the reproductive system was preparing for a pregnancy.) This is to be used as a general guide for healthy development. Development may vary among pregnancies due to mother's health. For more details and resources, check out these links:
Day 1: Sperm joins with the ovum to form one cell smaller than a grain of salt. Twenty-three chromosomes from each parent join to form every detail of human development: sex, hair, eye color, height, skin tone, personality, emotional make-up, and other inherited characteristics.
Day 3-4: The fertilized egg is rapidly dividing as it travels down the fallopian tube into the uterus where for the last two weeks the lining has been preparing to receive the zygote.
Days 5-9: The zygote implants in the lining and begins to draw nourishment from the lining.
Days 10-14, Week 2: The zygote splits into two sets of cells, half become the placenta, which provides nourishment for the fetus and the other will become the fetus. Placental chemicals and hormones prevent the women from menstruating.
Day 20: The embryo is now the size of an apple seed. The placenta and umbilical cord are now functioning. The foundations for the brain, spinal cord and nervous system are established.
Day 21, Week 3: The heart begins to beat.
Day 28, Week 4: The backbone and muscles are forming. Arms, legs, eyes and ears have begun to show. Hair has begun to sprout.
Day 30: The embryo is 10,000 times larger than the original fertilized egg. The heart is pumping increasing quantities of blood through the circulatory system. The placenta forms a unique barrier that keeps the mother's blood separate while allowing food and oxygen to pass through to the embryo.
Day 35, Week 5: Five fingers can be discerned in the hand. The eyes darken as pigment is produced. The embryo is now the size of a raspberry.
Day 40: Brain waves can be detected and recorded.
Week 6: The liver is now taking over the production of blood cells and the brain begins to control movement of muscles and organs.
Week 7: The uterus is about the size of a tennis ball. The embryo is moving continuously. The jaw forms, including teeth buds in the gums. The eyelids seal to protect the embryo's developing light-sensitive eyes.
Week 8: Now a little more than an inch long, the fetus has everything found in a fully developed adult. The stomach produces digestive juices, the kidneys are functioning and genitals have begun to form. Forty muscle sets operate in conjunction with the nervous system and the fetus responds to touch.
Week 9: Fingerprints are already evident in the skin. The fetus will curve its fingers around an object placed in its palm. The fetus weighs about 1/2 ounce and is developing fingernails and hair.
Week 10: The fetus can bend, stretch, make fists, open hands, lift its head, squint, swallow and wrinkle its forehead.
Week 11: The fetus is now two inches long. Urination occurs.
Week 12: The fetus now breaths amniotic fluid, sleeps, awakens, exercises, turns its head, curls its toes and opens and closes its mouth.
Week 13: Fine hair has begun to grow on the head, and sexual differentiation has become apparent.