The concept of when your due date is, is based on a gestational length established back in the early 1800’s by Franz Carl Naegele. He officially declared that pregnancy lasted 10 lunar months (10 x 28 days), counting from the first day of the last menstrual period) He assumed that the average woman had cycles that lasted 28 days and that she ovulated on Day 14 of her cycle. He used his data to come up with a mathematical calculation for due dates:
(LMP + 7 days) – 3 months) = Due Date
EX: ((January 1, 2010 + 7 days) – 3 months) = October 8, 2010
However, Dr. Naegele did not consider certain factors in his calculation. For example: Not every woman ovulates on Day 14. Other situations that he did not factor in were ethnicity, parity (how many successful pregnancies), prenatal care, better nutrition, and screening factors. One study indicates that we need to add 15 days to the Naegele EDD (estimated due date) for Caucasian, first time moms, and 10 days for Caucasian moms having subsequent children (multiparas). African American and Asian women tend to have shorter gestations.
Let’s bring in the ultrasound component into determining your due date. Ultrasound-determined due dates are not accurate. Even first trimester ultrasound measurements have an error bar of +/- 5 days; in the second trimester +/- 10 days, and +/- 22 days in the third trimester.
Few healthcare providers appreciate the limitations of ultrasound or clinical data. The due date should not be changed unless the discrepancy is more than two weeks, yet we see healthcare providers changing a due date by a few days, no trivial alteration if a woman will be induced when she exceeds a certain date. Some risk does accrue in healthy postdate pregnancies (notably meconium passage and big babies) but it does not follow that we should induce all women. If we try to eliminate postdate pregnancies on grounds of increased complications, should we not equally logically try to delay labor onset in the early-term group?
– Henci Goer, Obstetric Myths vs. Research Realities, Bergin & Garvey 1994
Carol Wood, Yale nurse-midwifery professor, came up with a method to calculate the due date that takes into account individual variations in the menstrual cycle as well as the effect of a woman’s having had previous pregnancies.
1. Add 1 year to the first day of the last menstrual period, then
For first-time mothers, subtract 2 months and 2 weeks
For multiparas, subtract 2 months and 2.5 weeks (18 days)
2. Add or subtract the number of days her cycle varies from 28 days
*1st-time mothers with 28-day cycles: LMP + 12 months – 2 months, 14 days = EDD
*Multiparas with 28-day cycles: LMP + 12 months – 2 months, 18 days = EDD
*For cycles longer than 28 days: EDD + (actual length of cycle – 28 days) = EDD
*For cycles shorter than 28 days: EDD – (28 days – actual length of cycle) = EDD
EDD: Estimated day of delivery
LMP: Last menstrual period
Excerpt from-Anne Frye, Holistic Midwifery Vol. 1, Labrys Press 1995
Most authorities agree that there are many ways to determine an EDD (estimated date of delivery) of a pregnancy, and that not just one factor should be used to determine the final due date. Other events to factor in are:
Quickening (first time mom feels the baby move)
Fetal heart tones heard through doppler
Fundal height (Measurement of the uterus done throughout pregnancy)
Keep in mind that due dates are estimates of when your baby will arrive. We generally consider the normal time frame to be two weeks before your due date, until two weeks after your due date.
Just remember, no matter how many times your neighbors, family, friends, and even strangers ask you when you are due, smile, because only the baby really knows.