When an ovum is fully mature, the ovarian follicle ruptures and the ovum is released. This process, known as ovulation, always occurs 14 days prior to the next predicted menses, or menstrual flow. Once released, the ovum is capable of being fertilized for approximately 36 hours. Sperm deposited in the vagina near the time of ovulation, may move through the cervix and uterus to the Fallopian tubes, which run from the ovary to the uterus, where fertilization normally occurs in the third of the tube that is closest to the ovary. Embryonic development begins with fertilization, in which the male germ cell, the sperm, fuses with the female germ cell, the ovum, to form a zygote. Based on the dating of pregnancy from the first day of the last menstrual period, pregnancy lasts approximately 280 days (10 lunar months or 9 calendar months). However, the actual duration of prenatal development is about two weeks less.
At the beginning of development the single-cell is barely visible to the naked eye. While rapid growth in size is a characteristic of prenatal development, the process of differentiation by which a single-cell transforms into an integrated system of tissues and organs comprising many types of cells, from blood, to brain, to bone, is truly remarkable. Nearly all of this cellular differentiation takes place during the seven weeks following fertilization, after which time nearly all the tissues and organs of the body are identifiable, and many of them are functioning at a elementary level. Thus, the single-cell is transformed into a structure having identifiable human characteristics in the short period of seven weeks.
Near the end of the first week the developing embryo adheres to the epithelial lining of the uterus, beginning the process of implantation. The only indication of an embryo at that time is a small clump of relatively undifferentiated cells. By the middle of the second week the cell mass changes into a flat plate with two distinct cell layers. During the third week, a third layer of cells form and a number of specific structures begin to appear: the precursor of the vertebral column; the first indication of the central nervous system; networks of blood vessel; and the preliminary development of a heart, appearing as a pair of adjacent vessels. The substantial development of specific organs begins in the fourth week, although the embryo still measures less than 2 mm (0.1 inch) in length. By the end of the fourth week, the embryo is more similar to the adult form, with an identifiable head, eyes, nose, ears, neck, trunk, and limbs, and a cardiovascular system, which though rudimentary, is a functioning system complete with a pulsating heart and circulating blood. Growth and differentiation continue through the fifth week. Most organ formation is completed through the sixth week. By the end of the seventh week the embryo, less than 25 cm (1 inch) long, looks like a miniature human. Active differentiation continues in the organs already present, few new structures appear, and blood from the maternal circulation begins, for the first time, to enter the placental circulation under arterial pressure.
After seven or eight weeks of development (considered as the ninth or tenth week of pregnancy), the developing child is arbitrarily defined as a fetus. Subsequent development is primarily a function of growth and cellular or subcellular differentiation. At six months the fetus is about 25 cm (10 inches) long and weighs about 227 grams (8 ounces). During the final three months, the developing child doubles in length and increases in weight about 16 times.
Click here for more information, and to view photographs of fetal development.
Grolier Interactive on-line Encyclopedia 2000
Compton's Interactive Encyclopedia 1995
First Trimester: physical changes and symptoms.
Second Trimester: physical changes and symptoms.
Third Trimester: physical changes and symptoms.
Pregnancy through your ages (teens, 20's , 30's, 40's, 50's): What age means to you and your baby.
General Information: Pregnancy and Reproduction Centre.