Figuring Out Your Due Date by the Conception Date

Calculating your estimated due date

Estimating your baby's due date by conception date isn't as easy as it sounds. Usually the exact day of conception isn't really known. Other factors can influence your due date as well.

Due Date by Conception Date: A Tricky Calculation

It sounds pretty simple, doesn't it? If you know when you had intercourse, you should be able to calculate your exact due date. Well, not exactly. There's a little more to it than that.

The reason it's so difficult to calculate your due date by conception date is because in many cases the actual conception date isn't known. Most women don't keep track of the days they have intercourse unless they are actively trying to become pregnant. Even in the cases where they do, it still may not accurately tell them when they conceived.

A Secret Meeting

This is due to two things. The first is when you ovulate. The second is when the sperm reaches the egg. Sperm can live up to three days after you've had intercourse. If you have not ovulated yet, the sperm has to wait until there is an egg to fertilize. If you do not ovulate in time, the sperm dies.So, if you know that you've had intercourse on a specific night, you may not have actually conceived until three days later. The exact day when the sperm and the egg finally meet is a secret; you have no way of knowing when it actually happens.

Still, you probably think this should only make a difference of three days or so in your calculations. What's three days in a 40 week pregnancy? How about the fact that only about five percent of babies follow this 40 week rule?

More Accurate Calculations

So, calculating your due date by conception date isn't very accurate. How can you get more accurate calculations? First, you are very unlikely to be able to calculate your exact due date, although it happens occasionally. Second, if you want a reasonably accurate calculation you need to be aware of a few facts.

Due Date Based on 28-Day Menstrual Cycle

Menstrual cycles vary from woman to woman. They can range from as few as 22 days to as many as 44 days. You need to know how long your average cycle is. If yours is an average of 28 days, you may find an online pregnancy calculator fairly accurate. If you add 280 days to the first day of your last period you will get the same result, based on that 28 day average. This calculation is based on the 40 week rule mentioned above.

Try this free due date calendar, which uses the first day of your last menstrual cycle to estimate when baby will arrive.

Due Date Based on Ovulation Period

Another helpful fact to know is when you ovulate. There are lots of ovulation predictors on the market. You can also track your basal body temperature to have a fairly accurate idea of when you will ovulate. This may give you a more accurate calculation than the 40 week rule.

Once you know your date of ovulation, add 266 days. This is more accurate because you know when you have actually ovulated as opposed to when the sperm is just sitting there hoping for a date.

Other Variables

There are a few other variables that can influence a baby's due date. The mother's age, her health, and even her race can all play a role in when her baby is born. A first time mom may carry her baby longer than a woman who is having her sixth child. A woman carrying multiples is likely to give birth sooner than a woman who has only one baby inside her womb.

There are other, more accurate methods of determining when a baby is due as well. Instead of calculating due date by conception date, ask your health care provider about using an ultrasound to determine your due date. Some doctors like to use HCG levels to calculate when a baby is due. These methods are very accurate and are also very safe. Your health care provider wants you and your baby to be healthy and happy. If you have any questions or concerns, don't hesitate to ask.


Figuring Out Your Due Date by the Conception Date