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Conception Calculator: When Did I Conceive?

As soon as you get a positive pregnancy test, the first thing you probably want to do is type “conception calculator” or “When did I conceive?” into Google. And if you do, you’ll find loads of online calculators that promise to give you your conception date.  But the truth is that online calculators (which use formulas) will only give you an estimate of your conception date.

Before we dive into understanding the complex process of conception, let’s take a look at how these conception calculators typically work.

When you’re trying to determine when your pregnancy began, the date you’re really looking for is implantation—when the fertilized egg burrowed into your uterine lining, thereby beginning your pregnancy. You can figure out this date using a couple of simple formulas (which we’ve also covered in our implantation calculator post).

If you know the date you ovulated, add 9* days.

Ovulation date + 9 days =  Implantation date

If you know the first day of your last menstrual period (LMP), add 23** days.

Date of LMP + 23 days =  Implantation date

*The average viable embryo implants at 9 days post ovulation.

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**14 (the average number of days between LMP and ovulation ) + 9 (the average number of days between ovulation and implantation) = 23 days.

Again, it’s important to note that the above formulas are just a rough guess based on when ovulation and implantation tend to happen on average for most women. For accurate information about your specific fertile window, tracking your cycle is really helpful.

When did I conceive? 

If you’re trying to figure out when conception occurred, it’s important to know that conception isn’t the whole story of when your pregnancy began. Rather, it’s a two-part process that includes both conception (fertilization of the egg after ovulation) as well as implantation.

What is ovulation?

During the follicular phase—the first half of your cycle—the fluid-filled sacs in your ovaries (follicles) that contain immature eggs will grow. One follicle will become dominant and begin secreting estradiol, which will cause a surge in luteinizing hormone (LH). This surge ruptures the follicle, releasing a mature egg, which then begins traveling through the fallopian tubes that connect the ovaries to the uterus.

The resulting egg release—triggered by the LH surge—is called ovulation. If you’re wondering when ovulation occurs, it varies for each woman, but on average, it happens around 14 days after the beginning of your period. (But again: cycle tracking will show you more precisely when you ovulate, instead of using a formula to arrive at an estimate.)

Once an egg is released, it must be fertilized within 12 – 24 hours, or the egg will no longer be viable. The healthiest sperm can remain viable inside a woman’s body up to five days when fertile cervical mucus is present. Since the window of fertility is quite short, using an ovulation calculator, or maintaining an up-to-date ovulation calendar or fertility calendar can help you keep track of the best time to get pregnant. (The short answer? Have sex two to three days before you ovulate!)

Finally, the now-fertilized egg must complete its journey to the uterus and implant in the uterine wall. Implantation generally occurs between 8 – 10 days after ovulation.

If any of these steps fail, the egg will die and be excreted during your next period, and the ovulation process will begin again for the next month.

When is my due date?

Once you’ve confirmed that you’re pregnant, of course, you’ll want to know when you can expect your baby’s arrival. Knowing your conception date helps with pregnancy tracking. More importantly, this study showed that having an accurate due date positively effects pregnancy outcomes, as it allows doctors to make better decisions, such as whether or not to induce labor. (Also, check out our post on figuring out how many weeks pregnant you are.)

Traditionally, your estimated due date (EDD) is determined by adding 280 days to the first day of your last menstrual period (LMP). However, this practice assumes your menstrual cycle is 28 days long, and that ovulation occurs on the 14th day after your period begins. Many women, however, do not operate on this cycle. Using the 28/14 model doesn’t account for irregularities in your cycle length, not knowing the exact date of your LMP, or variability in ovulation timing.

How accurate are conception calculators?

Conception calculators and due date calculators are great for providing a ballpark due date. But it’s important to remember these rough estimates don’t take the variability of your cycle into account.

When determining a fetus’s gestational age, doctors can utilize several different diagnostic methods. Perhaps the most ubiquitous involves using the date of your LMP. Other methods include dating the pregnancy from the LH surge, which happens just before ovulation; measuring the level of hCG, which is released into your body only once an egg successfully implants in the uterus; and measuring the fetus during first- and second-trimester ultrasounds.

Are online conception calculators different from the “wheel” one that my doctor uses?

When you visit your doctor to discuss your pregnancy, they may use a gestational calculator called a pregnancy wheel. Like many conception calculators, the pregnancy wheel uses the date of your LMP. By shifting one layer of the wheel to the appropriate LMP date, other notable dates (like your possible due date) are revealed.

The information provided by the pregnancy wheel and conception calendars is the same. However, while a pregnancy wheel helps to determine your EDD, it also provides additional information about the various stages of pregnancy, such as the estimated weight of the fetus and the optimal windows for various types of prenatal screening.

A few years ago, The American College of Obstetricians and Gynecologists reinvented the pregnancy wheel as an app, and now encourages the use of this app in place of the older physical, plastic wheel.

Why is my calculated due date different from the one on my ultrasound?

A conception calendar uses the date of your LMP to calculate your estimated due date. An ultrasound, however, determines the estimated gestational age of an embryo or fetus by taking biometric measurements. Usually, this involves measuring the crown-rump length (CRL) of the embryo or fetus. In the first trimester, embryo sizes are pretty consistent. Thus, by comparing the observed size of your embryo or fetus to the established norm, it is possible to determine its gestational age more accurately.

According to the clinical guidelines of the Society of Obstetricians and Gynaecologists of Canada, the due date derived from ultrasound is the best one for clinical use. (But it won’t tell you the exact date of conception because of factors such as biological variability in reproduction, fetal size, and development.)

Can a conception calculator be wrong?

As much as you’d love to know the exact timing of conception (and, consequently, your EDD), conception calculators can be wrong because of the amount of estimation involved. Having an irregular cycle, a longer or shorter than average follicular phase, or an early or late implantation date can all affect your due date.

A study published in the International Journal of Gynecology & Obstetrics found that only 5% of women actually gave birth on their EDD, regardless of the dating method, while 66% of births occurred within a week of their EDD.

Tara Avery

Tara Avery is a freelance writer and editor currently based out of Vancouver, British Columbia. When she's not traveling the world in search of inspiration, she's writing fiction, film, and non-fiction across a variety of genres.

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