Egg Meets Sperm and Fertilization Occurs. As soon as a sperm cell penetrates the egg, fertilization occurs, usually high up in the fallopian tubes. At the moment of fertilization the baby's gender is determined (some sperm produce males, others produce females). Right at the start, the fertilized ovum contains a full compliment of genetic codes: twenty-three chromosomes from mom, and twenty-three from dad. Occasionally, two eggs are fertilized by two sperm, resulting in fraternal twins. Less commonly, one egg is fertilized by one sperm and then divides in two, with identical twins as the result.
Implantation Occurs. By day seven, the embryo resembles a microscopic raspberry and implants into the lining of your uterus. As baby burrows into the blood-rich lining a few drops of bleeding or spotting may occur. This blooming ball of life, called a blastocyst – meaning, "sprout pouch," begins to organize into groups of several hundred cells. Some of these cells take root into the plush uterine lining; others arrange themselves in clusters and cavities, each with a different human destiny. The uterus, responding to the presence of the embryo, begins to form a primitive placenta, which transfers nutrients from mother's blood into the developing baby and facilitates disposal of the baby's waste products. As the placenta develops, it begins to produce human chorionic gonadotropin (HCG), a hormone that keeps the uterine lining in place and stimulates its growth by keeping the levels of estrogen and progesterone high. HCG is released into the mother's bloodstream at an increasing rate as the placenta develops. By the end of the second week, a pregnancy test will be able to detect HCB in the mother's urine.
Hormones Surge while baby and placenta grow. By week three, your menstrual period is late and you may suspect you are pregnant. Your rising hormone level is likely to cause you to begin feeling pregnant. Pregnancy hormones notify the ovaries not to ovulate again, and the ovaries, via hormonal messengers, notify the pituitary gland in the brain to no longer stimulate menstruation.
Within three weeks, what started out as a single cell has grown to millions of cells that now begin to differentiate into three types of cells: those that will become the nervous system, skin, and hair; those that will make up the gastrointestinal tract and those that will form the circulatory, genito-urinary and musculo-skeletal systems. By the end of the third week, a rudimentary heart tube begins to beat and circulate blood. You're just beginning to feel pregnant and already the baby making is well under way.
Baby Takes Shape.
During this week, baby grows to the size and shape of a curved grain of rice. An umbilical cord, containing three distinct blood vessels, appears. Along the outer rim of baby's tiny body, blocks of tissue stack up to form the backbone. Tiny buds, soon to become arms and legs, emerge from the body. The ball-like heart divides into chambers and pumps blood into already formed major vessels. Specialized ultrasonic equipment can even detect a regular heartbeat. Tiny pits now present in the baby's head mark the spots where eyes and ears will form. Lobes of baby's brain and a primitive spinal cord develop. Rudiments of future organs such as the trachea, esophagus, stomach, mouth, liver, gull bladder, and thyroid appear. Amazingly, by the time most mothers-to-be attend their first prenatal check-up, most of their baby's major organs are well on their way.