THE DUE DATE

The “textbook” pregnancy due date is based on a 28-day cycle, ovulation and conception occurring on day 14, and therefore 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.

Using our method, the day that you miss your period, you are 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 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 always look at how many weeks pregnant you are at each visit, so an accurate due date is essential.

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 every 9 weeks equals 2 months: so 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 that you are. Let's say that your due date falls on a Sunday. Then one Monday at your prenatal visit we teel you that you are 20 1/7 weeks pregnant.

This means that you have completed 20 full weeks of pregnancy, and 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!

Premature

Delivery before 37 weeks.

Full-term

This is from 37 to 41 weeks gestational age.

Post-Dates

Delivery after 41 weeks (a few old-school docs say after 42 weeks).

When will you deliver?

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