Pregnancy

Definition

Pregnancy is the condition of having a developing embryo or fetus in the body. The union of an egg (ovum) with sperm is called fertilization, or conception, and it is this union that produces the embryo. Pregnancy includes the period from conception to birth of the fetus, and usually lasts 10 lunar months (40 weeks/280 days), or nine calendar months—as measured from the first day of the last menstrual period (LMP). It is also referred to as a gestation period that consists of three trimesters. The trimesters are not equal, but are defined by different stages of a baby's development. The first trimester includes the first 13 weeks of pregnancy, or the first 12 weeks of life. The second trimester consists of weeks 14–26, and the third trimester is weeks 27–40.

Description

At the time of sexual intercourse, a man ejects millions of sperm into the woman's vagina. The sperm travel in all directions, propelled by their whiplike tails, and many swim through the cervix toward the uterus. A very small number of them may survive as long as 48 hours, but only one has to make it to the fallopian tube to meet the egg that has been released from an ovary. It takes approximately 30 minutes following intercourse for the sperm to meet the egg. One sperm penetrates the egg and its tail is shed, while the remainder of the sperm provides one-half of the genetic material of the future fetus—the other half provided by the egg. The fertilized egg then travels along the fallopian tube, arriving in the uterus four to five days later. Fluid secreted by the tube lining provides it with nutrition during its travels. After two to three days in the uterine cavity, the fertilized egg implants into the thick lining of the uterus. Implantation occurs at about day 22 of a normal menstrual cycle. If the fertilized egg were visible to the naked eye, it would appear to be covered with fine hairs, called villi. These villi become the densest where the blood supply is richest, and eventually form the placenta. The mother's blood moves slowly around these villi, permitting them to absorb food and oxygen, and to eliminate waste products. The placenta is completely formed and functioning by 10 weeks after fertilization. Between 12 and 20 weeks' gestation, the placenta weighs more than the fetus, because the fetal organs are not sufficiently developed to deal with the processes needed for nutrition.