What Is the Estimated Date of Confinement?

When a woman is pregnant, her date of confinement is generally her due date, or the date when she is expected to give birth. The doctor determines the woman's estimated date of confinement based on the first day of her last menstruation. A full-term pregnancy usually is 37 to 42 weeks, with the due date somewhere in the middle. The term "date of confinement" comes from the fact that women once were confined to bed near the end of pregnancy in an effort to prevent premature birth and to protect the health of the women and their babies.

Generally, a woman's estimated date of confinement is 280 days from the first day of her last menstrual period. Although this date is an estimation, unless there are complications in the pregnancy, the woman can use this date to acknowledge developmental benchmarks of the baby. She also can prepare for the baby's birth sometime near that date.

Many women are confused about why doctors use the date of a woman's last period to determine the date of conception. For most women, it is difficult to determine the exact date of ovulation, and for those who have irregular menstrual cycles, it can be almost impossible. If the woman does know the date of ovulation, it is difficult to determine the exact date of conception, because an egg is fertilizable for a minimum of 24 hours, and sperm can live in the woman's body for as long as three days.

For women who have irregular menstrual cycles, it might be necessary for a doctor to determine the estimated date of confinement using an ultrasound image. Ultrasounds, or sonograms, transmit high-frequency sound waves through the belly. The machine then converts the echoes into video and photographic images.

Doctors can use these images to determine the baby's age by measuring the size of the baby. Doctors also use ultrasound imaging to determine a baby's health, position and expected birth weight. Generally, a woman will receive two ultrasounds during the course of her pregnancy, unless a doctor suspects complications.

Complications often occur during the second and third trimester of a pregnancy and before the estimated date of confinement. Most doctors discover pregnancy complications early through prenatal testing. A doctor generally uses the estimated date of confinement to determine the appropriate time to perform prenatal tests.