How Is Pregnancy Due Date Calculated?

in Maternity

A woman who is pregnant for the first time may wonder how the pregnancy due date is calculated. The pregnancy due date is the date a pregnant mother expects to give birth. It is generally nine months after conception, although it can vary.

There are several methods for calculating pregnancy due dates, and they have become more precise over time. Many women now use pregnancy trackers and due date calculators in order to predict the baby’s due date.

Factors Affecting Pregnancy Due Date

Due to unavoidable factors, some babies are born much earlier than expected, and some are born long after the estimated due date. Both cases are normal.

The pregnancy due dates of first-time mothers tend to be different from those of pregnant women who have already had children.  This could be because the uterus of a first-time mother is not accustomed to pregnancy, unlike the wombs of women who have experienced pregnancy before.

The age, weight, and height of the mother also affect the estimated pregnancy due date. The mother’s nutrition and physical activity during pregnancy can impact the due date as well.

Mothers who engage in physical exercise and eat well during pregnancy often have their babies on or before the due date because their good health aids the flexibility of the cervix. On the other hand, mothers who are less active take longer to give birth because their dormant lifestyles negatively affect their contractions and cervix opening.

An Estimated Pregnancy Due Date Calculator

An estimated due date, usually referred to as EDD, is simply the best guess when a baby might be born. EDD is usually based on the day of conception. However, the exact day of conception may be tricky to determine, which is why due dates are just estimates.

A woman may conceive any day within her fertile window, so every woman has a seven-day time period when she can potentially conceive. So, most clinical officers estimate the date of conception within a week, plus or minus.

Pregnancy experts calculate the due date by adding 280 days to the first day of a woman’s last menstrual period, which is based on the average 28-day cycle. If a cycle is longer, the due date will be later and if a cycle is shorter, the due date will be sooner. The 280 days is only an approximation.


If a woman finds out she is pregnant on the day her period should start, she is four weeks pregnant. This is still an estimate because ovulation happens around the fourteenth day of the menstrual cycle, so there is a two-week window. So, once 280 days are added to the first day of the last period, also include about a week (more or less) from the EDD because the EDD is just an approximation.

Hello! I am Roselia Shi, mom to a feisty one-and a-half-year-old girl named Tara. We live in Newport, Arkansas. I enjoy solving everyday health and parenting problems. I have a university degree in clinical pharmacy and pharmacology. I started exploring science-based parenting soon after Tara was born. Parenting Science is a small attempt to share some useful information and insights about responsive parenting.

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