Pregnancy is made up of many stages. Most women know that gestation is divided into three trimesters, but it turns out that early pregnancy--the time from a woman's last menstrual period until the end of the embryonic phase--contains many discreet stages in and of itself, each of which encompasses a different set of symptoms for the expectant mother and developmental milestones for the baby.
Weeks 1 and 2
Oddly enough, pregnancy begins before conception: Gestational age is actually counted from the first day of a woman's last menstrual period. As a result, during weeks one and two of pregnancy, there are no developmental milestones for the baby at all, since the egg in question not only hasn't been fertilized, it hasn't even been ovulated! Still, important changes are taking place in the mother's body during these weeks. Increasing levels of the hormones estrogen and progesterone cause the lining of the uterus to grow spongy and fill with blood, while the hormone FSH stimulates an egg to ripen. Lauralee Sherwood, Ph.D., notes in her text, "Human Physiology" that the egg will be ovulated near the end of week two of pregnancy in response to a surge of the hormone LH. If sperm are present at the end of week two when the egg is released, the egg may be fertilized and become a zygote.
Weeks 3 and 4
During these weeks, the zygote--a scientific term for the organism from fertilization until implantation--is made up of a ball of rapidly dividing cells called a blastocyst. Some time during the fourth week, the blastocyst implants in the uterine wall and begins to form the placenta. During this time, many women don't even realize they're pregnant. Others may experience symptoms similar to those they'd expect prior to the onset of a menstrual period. Still others find themselves suffering a complete array of pregnancy symptoms.
Weeks 5 and 6
By the beginning of the fifth week, most women know they're pregnant--they've missed a period, and symptoms of early pregnancy have begun in earnest. The Mayo Clinic notes that women in the full swing of the first trimester experience breast tenderness, nausea, and fatigue. The baby, too, is undergoing tremendous changes during this time. The eyes and ears form during the fifth week, while the heart begins to beat during the sixth.
Weeks 7 and 8
The last two weeks of the embryonic phase of gestation bring about more changes in the baby's shape and size, resulting in an embryo that, for the first time, looks more human than tadpole. The seventh week brings further organ development and much more growth--seven-week embryos are almost an inch long, according to the American Pregnancy Association. By the end of the eighth week, the baby has all the organs of a fully developed human, but they're not all functional yet. The face, fingers and toes are beginning to emerge from previously amorphous bulbs, and the muscles are capable of contraction for the first time. The end of the eighth week marks an important transition for both baby and mother. For the baby, it means the end of the embryonic period, and graduation to fetal status. For the mother, it means that the somewhat risky phase of early pregnancy is at an end.
References
- "Human Physiology"; Lauralee Sherwood, Ph.D.; 2004
- Mayo Clinic: Early Pregnancy
- American Pregnancy Association: Stages of Pregnancy
