How Pregnancy Due Date is Calculated
From conception vs last menstrual period
The average duration of pregnancy is 38 weeks (266 days) from conception. Predicting the pregnancy due date based on conception is the most accurate way to calculate one’s due date, but usually the date of conception is not known. Therefore, the due date is generally calculated from the first day of the last menstrual period (LMP). In this case, 2 weeks are added to the calculation giving a total of 40 weeks (280 days). This is based on the assumption that ovulation/conception occurs on cycle day 14 in the “average” 28 day menstrual cycle.
From ovulation
Calculating the due date from LMP is subject to error since ovulation varies in its timing from the onset of menstruation among different women and from cycle to cycle. Many of Dr. Monteith’s tubal reversal patients know their date of ovulation from using an ovulation predictor kit (OPK) or keeping a basal body temperature (BBT) chart. Calculating the pregnancy due date from ovulation is more accurate than from the last menstrual period.
Our pregnancy due date calculator gives you the option of determining your due date from either the date of ovulation or the date of LMP. You can choose either of these dates in the calculator below.
Reliability of due date
The pregnancy due date is an simply an estimate of when pregnancy is at term and labor will begin. It is reasonably accurate within + or – 2 weeks. Labor begins on the calculated due date, however, in only a small percentage of cases.
Monitor your early pregnancy
The best way to monitor early pregnancy is with quantitative serum HCG assays. In a normally developing pregnancy, serum HCG levels double approximately every 2.5 days. You can use our HCG doubling time calculator and HCG graph to keep track of your serum HCG test results during the first 2 or 3 weeks after your missed period and positive pregnancy test. When serum HCG reaches 1500, we recommmend a vaginal ultrasound examination. By that time, a gestation sac should be visible in the uterine cavity.
Part of Dr. Monteith’s long term follow-up care after your tubal reversal is following your early pregnancy along with you and your local physician. To enable us to do this, please submit your completed pregnancy report form as soon as you have a positive home pregnancy test.
Pregnancy Information
- Pregnancy Signs and Symptoms – in collaboration with the National Institutes of Health.
- Pregnancy Tests – detect the presence of the hormone human chorionic gonadotropin (hCG).
- Calculate your pregnancy due date – from ovulation or last menstrual period.
- Pregnancy Report Form – Send this when you have a positive pregnancy test.
- Birth Report Form – Send this when you have your baby after tubal reversal.
- Early Pregnancy Monitoring – Why tubal reversal patients require this!
- Pregnancy Announcements – weekly updates from Dr. Monteith’s tubal reversal patients.
