By the time you miss your period and the result of a home pregnancy test turns out positive, you are likely already four weeks pregnant. Calculating your estimated due date at this time allows you to have an idea of when your baby will be born, though only 5 percent of women give birth on their actual due date, notes the American Congress of Obstetricians and Gynecologists. In addition to your calculation, your doctor or midwife may choose to have an ultrasound done early in your pregnancy to obtain more accurate information.
Start with the date of your missed period and think back to when your cycle started the month before, or the date of your last regular menstrual period. Most women have an average cycle length of about 28 days so the date of your previous period will likely be about four weeks before your missed period.
Use that date of your last menstrual period and count ahead on your calendar 280 days, or 40 weeks, to determine your estimated due date. For example, if your last menstrual period started on Nov. 1, you would count forward 40 weeks for a due date of Aug. 8.
Take the date of your last period, adding seven and then subtracting three months, as an alternate method. If your last period started on Nov. 1, you would add seven, making the date Nov. 8 and then use your calendar to count backward three months to Aug. 8.
Provide your information to your doctor and midwife and ask her for her calculations as well. As standard pregnancy due date estimations assume that you have a 28-day cycle, your medical provider may be able to provide a more accurate due date based on your individual experiences and cycle length.
Due date calculations only provide an estimation.