At the time of conception an egg is fertilized, thereby creating a unique new cell. The cell divides very quickly into many more cells and at about 1 week after conception this tiny mass of cells attach itself to the wall of the uterus (implantation). The placenta and amniotic sac soon develop. Most internal and external body parts are forming, including the fingers and toes.
A missed period may be the first sign of your pregnancy. You may notice that your breasts become sensitive, full and tender, and that the nipple and circular area around the nipple (areola) darken. As your uterus enlarges during the early part of pregnancy, it will press on your bladder, so that you will need to urinate more often. You may experience nausea and vomiting, and may also notice that you tire more easily.
When a woman's egg and her partner's sperm unite, the process is called fertilization. The process normally takes place in one of the woman's fallopian tubes and produces a single cell, called a zygote. This is the first cell that, in a few months, will become a fully developed baby. The genetic makeup (the characteristics that make this new individual different from anyone else in the world) is determined at the moment of fertilization. This time of growth and development is called gestation.
Within approximately one week of fertilization, the zygote attaches itself to the wall of the uterus in a process called implantation. The placenta and amniotic sac begin to develop. Within two weeks, that first cell will have multiplied into a cluster of many thousands of cells. The cluster is now known as an embryo, a Greek word meaning "to swell."
The embryonic phase of development lasts from about the third week of gestation until about the end of the eighth week. It is a crucial time for the baby-to-be, as most of his/her major internal and external systems, body organs, and parts are forming. Everything the mother ingests flows through the placenta to the embryo. Alcohol, drugs, and tobacco can be harmful to a developing baby. Knowing this should remind you of the importance of watching what food you eat and medications you take.
By the end of the eighth week of gestation, the embryo is about 1 inch in length and weighs about 1/30th of an ounce. It's now called a fetus, a Latin term meaning "young one." It has developed into a recognizable human being.
By the time the fetus is 12 weeks of gestation, he/she is very active, but still so small the mother does not feel the movement. Typically, a fetus of this age is about 3 ½ inches in length and weighs about 1 ½ ounces. Sex organs have now developed. The placenta is completely formed and is producing estrogen and progesterone, two hormones that help maintain a pregnancy. The umbilical cord has started to circulate blood from the mother to the fetus, providing nourishment.
Reprinted from Her HealthCare.