Generations of midwives, doctors and mothers have estimated the expected delivery date of babies. Central to measuring progress, critical to decision making and a primary concern of the mother and all affected is the due date. Indeed, the first question a mother-to be hears is usually, "When is it due?" Everyone wants to know, but only the baby knows for sure.
Identification
The due date is the estimated date of delivery or EDD. Only one in 20 women gives birth on that exact date. Typically, a pregnancy lasts 38 to 42 weeks, and most women give birth within two weeks before or after their due date.
History
For centuries, people have estimated due dates based on a normal pregnancy of 10 lunar or nine calendar months. Franz Karl Naegele, a German obstetrician, developed a standard way of estimating a due date and published it in 1830 as a guide for midwives. In his method, start with the first day of the last menstrual period, add a year, subtract three months, and add seven days to that date, according to the British Journal of Obstetrics and Gynaecology. Assumptions included average menstrual cycles, average months and average length of gestation. Later variations of Naegele's rule provided more precision. Beginning in the 1970s, ultrasound scans have measured the size of developing embryos directly to estimate gestation age with greater accuracy.
Function
The gestational age and due date have considerable medical implications. Establishing a time line allows for monitoring normal growth and development of the infant. The danger of structural defects caused by teratogens, or harmful agents that interrupt the normal development of the limbs and organs, is greatest early in embryonic development and lasts for several months, according to John Santrock in "Life Span Development." Critical periods of development are associated with each organ and system. How things like maternal diseases and conditions, nutrition, stress, drugs and environmental hazards influence prenatal development depends on the stage of embryonic and fetal development. In addition, labor is often induced seven to 14 days past the expected date of delivery to alleviate concerns related to failing placental function or other complications.
Calculation
A normal pregnancy lasts approximately 280 days or about 40 weeks, starting with the first day of the last menstrual period, according to the American Congress of Obstetricians and Gynecologists. A normal range is 259 days to 294 days, or 38 weeks to 42 weeks. The 40 weeks of pregnancy are divided into three trimesters, each lasting about about 12 to 13 weeks, or three months.
Considerations
The estimated date of delivery is useful but only approximate. It can guide the doctor in checking a baby's growth and the progress of the pregnancy and provides a rough idea of when the baby will be born. Women usually go into labor and deliver within the two weeks surrounding the estimated due date, depending on factors such as whether this is the first pregnancy, the time between pregnancies and variations in menstrual cycles that can affect time of conception.
