Pregnancy begins when an egg is fertilized by a sperm. For about 9 months, a pregnant woman’s body provides a protective, nourishing environment in which the fertilized egg can develop into a fetus.
If a menstrual period is a week or more late in a woman who usually has regular menstrual periods, she may be pregnant. Typical symptoms of pregnancy include the following:
Enlarged and tender breasts
Nausea with occasional vomiting
A need to urinate frequently (but without pain while urinating)
Unusual fatigue
Changes in appetite
Home pregnancy tests
When a menstrual period is late, a woman can use a home pregnancy test to check whether she is pregnant. Home pregnancy tests detect human chorionic gonadotropin (hCG) in urine. Human chorionic gonadotropin is a hormone produced by the placenta early in pregnancy.
Results of most types of home pregnancy tests are very accurate, but they are not 100% accurate. If results are negative but a woman still suspects she is pregnant, she should repeat the home pregnancy test a few days later.
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Laboratory pregnancy tests
At a clinic or hospital, doctors determine whether a woman is pregnant by testing a sample of urine or sometimes blood.
Due date
The due date (also called date of delivery) is usually estimated to be 40 weeks from the first day of the last menstrual period. The pregnancy actually begins when an egg is fertilized by sperm (conception), but this date is more difficult to determine than the first day of a period, and that is why the period date is used rather than the actual date of conception.
The ovaries usually release an egg (called ovulation) about 2 weeks after a woman’s menstrual period starts. Fertilization, if it happens, usually occurs shortly after ovulation. Consequently, the embryo is about 2 weeks younger than the number of weeks traditionally assigned to the pregnancy. In other words, a woman who is 4 weeks pregnant is carrying a 2-week-old embryo. If a woman’s periods are irregular, the actual difference may be more or less than 2 weeks.
Pregnancy lasts an average of 266 days (38 weeks) from the date of fertilization (conception) or 280 days (40 weeks) from the first day of the last menstrual period if a woman has regular 28-day periods.
To estimate the due date, after pregnancy is confirmed a woman’s doctor asks her when her last menstrual period was. The doctor calculates the approximate date of delivery by using a digital due date calculator or a manual pregnancy wheel device. Another method is counting back 3 calendar months from the first day of the last menstrual period and adding 1 year and 7 days. For example, if the first day of the last menstrual period was January 1, counting back 3 months is October 1, then adding 1 year and 7 days results in a calculated due date of October 8 in the coming year.
Often, an ultrasound is done to confirm the gestational age. If the gestational age determined by the ultrasound, especially an ultrasound done early in pregnancy, is different from the estimated due date based on the last menstrual period, the doctor may adjust the estimated due date.
How far along a pregnancy is (called gestational age) is noted in weeks, with a total of about 40 weeks in a typical pregnancy. Sometimes, a doctor will note both weeks and days when a gestational age needs to be precise (such as, 30 weeks and 2 days).
Pregnancy is divided into three 3-month periods, based on the date of the last menstrual period:
First trimester: Up to 13 weeks and 6 days of pregnancy
Second trimester: 14 weeks to 27 weeks and 6 days
Third trimester: 28 weeks to delivery
Delivery up to 3 weeks earlier or 2 weeks later than the estimated delivery date is considered normal. Delivery before 37 weeks of pregnancy is considered preterm; delivery after 42 weeks is considered postterm.
