How to know your conception date and due date

Pregnancy lasts an average of 280 days (40 weeks) from the first day of your last menstrual period (LMP).

The first day of your LMP is considered day 1 of pregnancy, even though you probably didn't conceive until about two weeks later (fetal development lags two weeks behind your pregnancy dates).

Calculating your due date isn’t an exact science. Very few women actually deliver on their due date, so while it’s important to have an idea of when your baby will be born, try not to get too attached to the exact date.

How to calculate your due date

If you have regular 28-day menstrual cycles, there are two ways to calculate your due date.

1. Naegele's Rule

Naegele's rule involves a simple calculation: Add seven days to the first day of your LMP, then subtract three months.

For example, if your LMP was November 1, 2015: add seven days (November 8, 2015) subtract three months (August 8, 2015) change the year, if necessary (to the year 2016 in this case)

The due date would be August 8, 2016.

2. Pregnancy Wheel

The other way to calculate your due date is to use a pregnancy wheel. This is the method that most doctors use.

It's very easy to estimate your due date if you have access to a pregnancy wheel. The first step is locating the date of your LMP.

When you line up that date with the indicator, the wheel will display your due date.

It's important to remember that the due date is only an estimate of when you will deliver your baby. The chances of actually having your baby on that date are very slim.

This is more common than you would think. Luckily, there are ways to figure out your due date when you can't remember the first day of your LMP.

If you know you had your LMP during a particular week, your doctor can estimate your due date accordingly. If you have no idea when your last period was, your doctor may order an ultrasound to determine your due date.

However, some women have cycles that are consistently longer than the average 28-day cycle. In these cases, a pregnancy wheel can still be used, but some simple calculations will be necessary.

The second half of a woman's menstrual cycle always lasts for 14 days. This is the time from ovulation to the next menstrual period. If your cycle is 35 days long, for example, then you probably ovulated on day 21.

Once you have a general idea of when you ovulated, you can use an adjusted LMP to find your due date with a pregnancy wheel.

For example, if your menstrual cycle is usually 35 days long and the first day of your LMP was November 1:

add 21 days (November 22) and subtract 14 days to find your adjusted LMP date (November 8)

After you calculate your adjusted LMP date, simply mark it on the pregnancy wheel and then look at the date where the line crosses. That is your estimated due date.

Some pregnancy wheels may allow you to enter the date of conception --which occurs within 72 hours of ovulation-- instead of the date of your LMP.

Be that as it may, your doctor might change your due date if your fetus is significantly smaller or larger than the average fetus at your particular stage of pregnancy.

And as a result, your will order an ultrasound to determine the age of your baby when there’s a history of irregular periods, when the date of your LMP is uncertain, or when conception occurred despite oral contraceptive use.

What is ultrasound

An ultrasound allows your doctor to measure the crown-rump length (CRL), or length of the fetus from one end to the other. During the first trimester, this measurement provides the most accurate estimation for the age of the baby.

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