The “textbook” pregnancy due date is based on the standard 28-day cycle with ovulation and conception occurring on day 14. The due date is 40 weeks (280 days) from the first day of the last menstrual period (called the LMP), or 38 weeks (266 days) from conception.

We call the due date the EDD, or estimated date of delivery.

Using this method, on the day that you miss your period you are already 4 weeks pregnant.

We determine your correct due date using a combination of menstrual timing, ovulation timing (if known),
dates of pregnancy tests and the results from early ultrasounds.

If the due date based on the LMP is different than the due date based on ultrasound, we need to decide which due date is correct, and this is done by the doctor at the OB consult visit. We look at how many weeks pregnant you are at each visit, so an accurate due date is essential.

Even when we know that you will deliver before the due date (such as with twins) we still use the due date to determine how far along the pregnancy is.

Here is a quick Due Date calculator:

Due Date Calculator
First day of your last menstrual cycle
Normal length of your menstrual cycle
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How many months?

How many months pregnant are you? Many people think that 4 weeks equals a month, but this is not quite accurate, since a month is closer to 4 ½ weeks. It is better to calculate that 9 weeks equals 2 months.

Thus, at 18 weeks you are 4 months, at 27 weeks you are 6 months and at 36 weeks you are 8 months pregnant.

How many weeks?

Using the Obstetric Wheel, our mini-calculator, we plug in your due date and determine how many weeks pregnant you are. This called your gestational age.

Let's say that your due date falls on a Sunday. Then on Monday at your prenatal visit we tell you that you are 20 1/7 weeks pregnant. This means that you have completed 20 full weeks of pregnancy plus one more day. Another way to say this is that you are in your 21st week. These two things mean the same thing.

But, you are not 5 months pregnant yet. Why not? (see above). And, to add a bit more confusion, you are not
5 months pregnant yet but you are in your 5th month!


Delivery before 37 weeks.

Full-term (now called Early term and Full term)

Full term used to be defined as from 37 to 41 weeks gestational age.

In November 2013, these definitions were expanded.
Early term refers to the gestational age from 37 weeks to 38 weeks, 6 days and
Full term is from 39 weeks to 40 weeks and 6 days.

Post-Dates (now called Late term and postterm)

Late term is 41 weeks to 41 weeks and 6 days.
Postterm refers to gestational age 42 weeks and beyond.

When will you deliver?

The due date is the best guess we have for when someone will actually deliver. Statistically 75-80% of patients deliver full-term, ten to fifteen percent premature, and 5 to 10 percent postterm.

IVF Patients

Fresh Cycle IVF - When you conceive during the same cycle as your egg retrieval, we use the date of the egg retrieval as your date of conception in order to calculate your due date.

Frozen Cycle IVF - When a frozen embryo is thawed out and then transferred with a resultant successful pregnancy, we need to know how many days the embryo was when it was thawed and transferred. If a 5-day old frozen embryo is transferred, we count backwards 5 days to come up with a "theoretical" date of conception and use that to determine your actual due date.