Pregnancy is the term used to describe a woman's condition during a period of approximately 9 months, during which one or more fetuses develop inside the woman. Typically, childbirth occurs after about 38 weeks of fertilization, or about 40 weeks after the last menstrual period. The World Health Organization defines the normal pregnancy period as between 37 and 42 weeks. During the first visit to an obstetrician-gynecologist, the doctor usually provides an estimated due date (based on ultrasound) for the baby's birth, also known as the due date. Alternatively, the due date can also be estimated based on the person's last menstrual period.
While the due date can be estimated, the actual duration of pregnancy depends on multiple factors, including age, length of previous pregnancies, and the mother's weight at birth. However, there are more factors that affect the natural variation in pregnancy duration that are not well understood. Studies have shown that less than 4% of births occur on the specified due date, 60% occur within a week of the due date, and nearly 90% occur within two weeks of the due date. Therefore, while it is possible to have some confidence that a person's baby will be born in about two weeks from the due date, it is not guaranteed.
Pregnancy can be detected either by using pregnancy tests or by women themselves by observing a number of symptoms, including the absence of the menstrual period, an increase in basal body temperature, fatigue, nausea, and increased urination. Pregnancy tests include the detection of hormones that act as vital signs of pregnancy. Clinical blood or urine tests can detect pregnancy from six to eight days after fertilization. While clinical blood tests are more accurate and can detect precise amounts of the pregnancy hormone (which is present only during pregnancy) earlier and in smaller amounts, they take longer to evaluate and are more expensive than home pregnancy tests. Clinical urine tests are also available, but these tests are not necessarily more accurate than home pregnancy tests and can be more expensive as well.
There are several factors to consider during pregnancy, many of which depend heavily on each individual's condition, such as medications, weight gain, physical activity, and nutrition.
Taking certain medications during pregnancy can have permanent effects on the fetus. In the United States, drugs are classified into categories A, B, C, D, and X by the Food and Drug Administration (FDA) based on the potential benefits versus fetal risks. Drugs that carry positive benefits for the mother and low risks to the fetus are classified as category A, while drugs with established and significant fetal risks that outweigh potential benefits to the mother are classified as category X. Pregnant individuals should consult their doctor about any medications they plan to use during pregnancy.