How To Calculate Your Due Date Based On Ovulation
Calculating your due date based on ovulation is simpler than you'd think. Read on to learn more with Natalist.
By OBGYN Dr. Kenosha Gleaton
If you’re hoping to conceive soon or if you think you may be pregnant, you’re probably curious as to when your due date might be. There are a few ways to determine an estimated due date based on your menstrual cycle.
What is a due date?
On average it takes about 40 weeks for a baby to fully develop in the womb, although a normal pregnancy can range from 38 to 42 weeks. When you’re given a due date, you’re being given an estimate of what day your baby will complete its gestation and be ready to enter the world, AKA your baby’s birth date! The due date is an important marker for yourself and your healthcare provider to appropriately prepare for and monitor your baby. Having a due date in mind can give you an estimate of when you’ll need to have everything prepared to bring your baby home, and ensure that your baby is developing properly in the womb.
What does ovulation have to do with the due date?
We’ll talk more about how due dates are calculated later, but first let's talk about what ovulation has to do with it. Ovulation, as you might remember, is the period of time in which an egg is released from an ovary and awaits fertilization. There is about a six-day window every month when the chances of conceiving are the highest. If sperm is present and fertilizes the egg, conception has occurred! If all goes well, an embryo will continue to grow for the next 38 to 42 weeks. Ovulation is the starting point for conception where sperm and egg meet, and therefore is one of the closest ways to pinpoint when the expected due date will be.
How soon after ovulation can you test for pregnancy?
How soon will a pregnancy test actually show a positive result? It’s important to wait an appropriate amount of time after ovulation to take a pregnancy test so that you aren’t given a false result. Most early detection pregnancy tests can give you an accurate result up to five days before your period is supposed to start! Depending on how long your cycle usually is, this would be around nine or so days after you ovulate. If you want to know when you’re ovulating, use ovulation tests to track luteinizing hormone (the ovulation hormone) in your urine! There are also some signs of ovulation you may notice, such as bloating, egg white discharge, and ovulation pain.
If you are actively trying to conceive or have gotten a positive pregnancy test, make sure you stock up on prenatal vitamins and make an appointment with your healthcare provider as soon as possible!
How is your due date calculated?
So how is the due date actually calculated? There are a few ways to estimate a due date:
Knowing the date of your last menstrual period
Knowing the date of your last menstrual period (LMP) can be a helpful way of estimating when conception occurred without having to know the actual date of ovulation. If you are not someone that tracks ovulation, or you weren’t taking ovulation tests in order to conceive, then your last menstrual period can give you and your doctor a general idea of when conception happened. Ovulation typically occurs on day 14 of the menstrual cycle (although this varies by individual), which means conception likely happened about two weeks after the first date of your last period (day one of your cycle). Generally, the due date will be about 40 weeks after the first day of menstruation.
Knowing the date of conception and/or ovulation
Knowing the date(s) you had sex as well as when you ovulated can also help determine a due date. Sperm can live inside the body for up to five days, so the actual date you had sex may not be a great indicator, but if you were taking ovulation tests, induced ovulation, or if you just have a fairly regular cycle, you may be able to pinpoint about the time of conception. Counting ahead 38 weeks from this date will be your due date.
Find out other ways to track ovulation.
Perhaps the most accurate way to determine gestational age and estimate a due date is through ultrasound measurements. This can be done throughout pregnancy but studies show that early measurements in the first trimester are likely the most accurate when estimating due dates. More specifically, measuring the crown-rump length (CRL), or the length from the crown of the head to the bottom of the torso, has an accuracy of ±five to seven days. The reason these early measurements are the most accurate is likely due to the lack of genetic variation at such an early stage.
There are various due date calculators online that typically work by asking for your LMP and sometimes your cycle length. These can give you a good estimate of what a due date might be, but remember to see your healthcare provider as soon as you can to get an ultrasound done for the most accurate estimated due date.
Calculating a due date after IVF
If you conceived through IVF, your doctor can give you a better estimate of when your due date may be based on the type of embryo transfer and other factors. In general, this can be calculated by:
Embryo transfer date and age of embryo
The date of embryo transfer and age of embryo used is another potential way to estimate the due date. There are multiple types of embryo transfers, including using a day three embryo transfer or day five embryo transfer, which can vary the estimated due date by a few days. The ACOG estimates that an embryo transfer with a day three embryo would have an estimated due date of 263 days later, around 37 to 38 weeks. Similarly, a day five embryo transfer has an estimated due date of about 261 days, a little over 37 weeks.
There are some online pregnancy calculators available for IVF pregnancies as well if you’re interested in seeing an estimated date, but your healthcare provider can give you a better estimate at a follow up visit, likely two weeks after your transfer date.
Can a due date be changed?
Yes, a due date can change with further measurements or monitoring. It’s not likely that a due date will change after an initial ultrasound is done during the first trimester, but if your healthcare provider first determines an estimated due date based on LMP or conception date alone, there is a chance the date will be changed by a few days to a week. Although average gestational age at birth can vary, pregnancies are still considered full term after 37 weeks, so earlier or later delivery is always a possibility. Your provider will also have a conversation with you if they think an estimated due date was incorrect or should be changed, so you will always be in the loop!
How accurate is the due date?
Due dates are just estimates of when birth may occur, but aren’t 100% accurate. Many factors can impact the date of delivery, including having multiples, injury, complications, or induction of labor. We do know that due dates estimated from an ultrasound during the first trimester have an accuracy of ±five to seven days, meaning estimated due dates are usually within a week of when actual delivery will occur. Talk with your doctor about any concerns you may have surrounding your estimated due date, and be sure to check out Natalist for more information and products to help you welcome your baby into the world!
A normal pregnancy can range from 38 to 42 weeks.
A due date is an estimate of when a baby will be born, and is a helpful date for the provider and the parents to prepare for and monitor the baby.
Ovulation is the starting point for conception, which is why it’s an important marker of conception when trying to calculate a due date.
Most early pregnancy tests can detect the presence of the pregnancy hormone as early as five days before a missed period.
A due date can be calculated using the date of the last menstrual period, the date of conception, or through tools like an ultrasound.
If you conceived through IVF, the due date can be estimated by your embryo transfer date and age of the embryo used.
Measuring the crown-rump length of an embryo during the first trimester is the most accurate way to find the gestational age and give an estimated due date.
A due date can be changed if measurements are pointing towards a different gestation age than originally thought.
Multiples, injury, illness, and induction of labor can all have an impact on the due date. Due dates are only estimates and may not be completely accurate, although the use of sonography decreases the margin of error to about five to seven days.