When did I get pregnant?

Getting Pregnant

Once you’ve had confirmation of your pregnancy by means of a positive urine or blood test, one of the first things on your mind is “when did I get pregnant?”  Knowing when you got pregnant is the first step in finding out when your due date is.  Knowing your due date accurately is important in many ways.  It can help you figure out when to have certain tests during the pregnancy and can help you determine exactly when to expect your baby to arrive.
when did I get pregnant

There are several ways you can calculate your due date and find out when you got pregnant.  Here are some typical ways women can find out when they got pregnant:

Counting from the first day of the last menstrual period

 when did i get pregnantThe first day of the last menstrual cycle is one of the most common ways to determine when you will have your baby. As the typical pregnancy lasts about 280 days or 40 weeks from the first day of your last menstrual period, you can count 280 days out from the first day of your last menstrual period and this will give you a good estimate as to when your baby is due.   Remember that, even though you aren’t technically carrying the fetus until you ovulate, which is usually 14 days from the first day of your last menstrual cycle, the pregnancy is said to begin two weeks prior to ovulation or the first day of your last menstrual cycle.  Don’t get confused and count 280 days from the date of conception as you will be off on your calculations by two weeks.  Inaccurate due dates can result in your having to have an unnecessary induction or having your baby too early because you didn’t know exactly when you got pregnant.

Using a due date calculator

If you don’t want to get out a calendar and actually count every day forward from the first day of your last menstrual cycle, you can go on line and put your dates into an automatic pregnancy calculator.  There are numerous web sites you can go to in order to find out when to expect your baby.  The March of Dimes site has a good due date calculator. All you need is the first day of your last menstrual cycle and the computer will do the rest of the work.   These pregnancy calculators are pretty accurate and you can find out your due date without having to go through the arduous task of counting out 40 weeks on a calendar.

If your cycles are irregular

If you have irregular cycles, especially cycles that aren’t 28 days in length, you might be inaccurate as to when you will have your baby.  When your cycle is longer than 28 days, it usually means you are ovulating late and using your first day of your last menstrual cycle will not give you the correct due date.  If you used an ovulation detection kit and know the exact day you conceived, you can get out a calendar and count 38 weeks forward to get your exact due date.  This translates into not 280 days but instead, you’ll need to count out 266 days forward from the day you ovulated according to the pregnancy calculator.

A pregnancy due date wheel

Your doctor, obstetrician, or midwife will have a pregnancy wheel in their office.  Such a wheel has all 40 weeks of the pregnancy on a wheel that you rotate to determine when your due date is supposed to be.  It is only accurate if you have 28 day cycles but will quickly let you and your doctor know when you will have your baby.  This is the most common way women find out when their due date is.

A simple calculation

If you don’t have a calendar or if you don’t want to count 40 weeks from the first day of your last menstrual cycle, you can determine your due date to a reasonable degree by doing this simple calculation:  Take the first day of your last menstrual cycle and subtract 3 months from that date.  Then add a week. This will give you an accurate measure of when your baby will be born. For example, if the first day of your last menstrual cycle was June first, you first back up and count three months before June first.  This gives you March first.  Then add 7 days, giving you a due date of Marth the 8th.  It is a simple calculation you can do in your head.  Like other methods of determining your due date, it depends on having a menstrual cycle that is 28 days in length.  If you have longer cycles than that, this will be an inaccurate way of determining your due date.

Using an ultrasound

 when did i get pregnant

An early ultrasound is a good way to determine when your baby is due.  If you have an ultrasound within about the 6th week of pregnancy, your actual due date can be calculated within about 5 days’ accuracy.  If you have a later ultrasound, the accuracy of the due date goes way down because babies are different sizes and, by the time you reach the 12th week of pregnancy, an ultrasound is only accurate within about a week, give or take.  If you have an even later ultrasound, such as at the twentieth week of your pregnancy or later, the due date can be off by as many as 2 to 3 weeks.  This is why you should have an ultrasound for dating the pregnancy that is as early as possible.  It isn’t common to have a different due dates, such as a due date based on your first day of your last menstrual cycle and an early ultrasound estimation.  If the ultrasound is done and indicates you are about six weeks pregnant or sooner, you should go by the ultrasound due date and not any calendar or wheel calculation.  This is because not every woman has a 28-day cycle and the ultrasound will be more accurate than using the wheel.

It is crucial to get a due date as soon as possible during the pregnancy.  Trying to think back as to your first day of your menstrual cycle gets fuzzier with time and you may not even remember when your last menstrual period was if you wait several months to calculate your due date based on your menstrual cycle unless you use a calendar and actually write down the date of your last menstrual cycle.

Women with long menstrual cycles have the hardest time determining their due date because the first day of the last menstrual cycle isn’t an accurate way of determining your actual due date.  A long menstrual cycle means that you’re a late ovulatory and you usually have to determine your ovulation date by subtracting 14 days from the date you expected to get your period and then counting 38 weeks from that day or by counting forward 266 days from the date you think you ovulated.

Knowing your due date is extremely important.  It helps the doctor know when to expect your baby and dictates when you have various testing during the pregnancy.  For example, if you have an alpha fetoprotein level checked for Down syndrome and don’t know when in your pregnancy you are to an accurate degree, you will not be able to make a determination as to the meaning of the alpha fetoprotein level because it is highly dependent on what week of pregnancy you are in.

The same holds true of the test for gestational diabetes.  If you are off on your dates, you might have the test outside of the 24 to 28-week interval and the accuracy of the gestational diabetes test will be suspect.


If you have several ultrasounds during the pregnancy, the technician will recalculate your due date based on the size of the embryo or fetus size.  Don’t be fooled by changing your due date based on this type of measurement.  Because the calculation of your due date is less accurate the further along in your pregnancy you are, those ultrasound-guided due date estimates can be extremely inaccurate if you decide to go about determining you’re your due date from an ultrasound done in the later parts of your pregnancy.

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