American Institute of Ultrasound in Medicine - AIUM.org
At the beginning, calculating your baby-to-be's due date is no easy task! To figure out your baby's expected arrival day, your health care provider will ask you when your last menstrual period (LMP) began. For most women, the average menstrual cycle lasts 28 days long, but normal range falls anywhere between 21 to 35 days in adult women (and from 21 to 45 days for young teens.
Sometime between days 11 and 21, your body will begin a process that might lead to pregnancy. During ovulation, an egg is released from your ovary and travels into the Fallopian tube. At the same time, the lining of the uterus prepares for a potential pregnancy by thickening. For about 12-24 hours that egg might be fertilized in the Fallopian tube. If sperm and egg come together -- you're pregnant! (Even though the egg is fertilized, it may be days before your pregnancy test would read positive.) But if the egg doesn't encounter sperm it continues to the uterus. In the absence of a fertilized egg, the uterus sheds its lining along with the egg, called menstruation.
While most women know the date when their last period began, most wouldn't know the day they ovulated. For that reason, health care providers use the LMP as a starting point for guesstimating the day of your baby's arrival. The most accurate way to figure out her arrival date is with an ultrasound, but not all health care providers request an ultrasound during the first trimester of pregnancy.
Terms to Know
Ovulation: The point at which the hormones in a woman's body signal the ovaries to release an egg into the Fallopian tube. This egg can then either be fertilized by a sperm or continue to the uterus, where it is released from the body during menstruation.
Important Information About Your Pregnancy
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Images courtesy of the American Institute of Ultrasound in Medicine (AIUM.org).