Your due date is calculated by adding 280 days (40 weeks) to the first day of your last menstrual period (assuming a 28 day cycle).
Note that your menstrual period and ovulation are counted as the first two weeks of pregnancy.
Use our Pregnancy Calculator to calculate these dates for you.
If the mother has a regular period and knows the first day of her last menstrual period, gestational age can be calculated from this date.
When people discuss how far along in pregnancy someone is, they tend to do so in general terms: “She’s in the first trimester” or “She’s four months along.” However, many people have only a vague idea of what those phrases really mean.
Medical professionals use a standard set of up to three methods to date pregnancies: last menstrual period, ultrasound, and a physical exam.
Most are born within a week either side of the estimated due date.A woman is usually able to get pregnant for about 5 days each month, when ovulation occurs.On average, ovulation occurs 12 to 16 days before the menstrual period begins.One reason for not dating pregnancies from the day of conception is that we cannot know that day exactly (excluding cases of assisted reproduction), but we can know the first day of LMP, based on what a patient reports to us.Also, when providers all use the same LMP date, we are using one system that provides a standard convention.