When you tell people you’re pregnant, what is the first question they usually ask? It’s typically, “When are you due?” It’s like that was also among the top questions you asked yourself after learning you were pregnant.
In this article, we’ll discuss pregnancy and how to determine your estimated due date.
Understanding Your Cycle and Conception
Each month, your body prepares for a possible pregnancy. This is a hormonal process referred to as your menstrual cycle or “cycle’, and it starts on the first day you get your period and ends on the day before your next period.
A typical menstrual cycle lasts anywhere from 21 to 35 days, but the average length is 28 days. Your cycle has four phases: menstrual, follicular, ovulatory, and luteal .
- Menstrual Phase: You have your period during the menstrual phase.
- Follicular Phase: The follicular phase begins on the first day of your period and lasts throughout the first half of your cycle. Hormones cause your ovary to get eggs mature for and ready ovulation.
- Ovulatory Phase: The ovulatory phase divides your cycle’s first half (follicular phase) and second (luteal phase). It marks the point when a mature egg is released from your ovary. The egg can survive and be fertilized by sperm for up to 24 hours.
- Luteal Phase: The luteal phase is the second half of your cycle and ends when your next period starts. The released egg travels through the fallopian tube to the uterus during this phase. At the same time, hormones prepare your uterus lining for a fertilized egg to implant. If sperm is already present in the fallopian tube or becomes present within 12-24 hours, the egg can be fertilized, marking the moment when conception occurs .
Your Due Date Based On a Conception Date
As you can see, conception occurs when a sperm fertilizes an egg during the second half of your cycle, in the luteal phase. So you might be wondering how many days during your cycle there is a possibility of becoming pregnant.
Once you ovulate, an egg can survive up to 24 hours, and sperm can live in your body for up to five days. This means there can be as long as six days in the middle of your cycle that you can become pregnant.
Remember that hormones cause your sex drive to be at its highest while you’re ovulating. Also, keep in mind that you can get pregnant while you’re breastfeeding, using birth control, or having vaginal bleeding.
Sometimes, women know approximately the date they conceived, also known as the conception date. This can be pinpointed with the greatest accuracy if you only had unprotected sex once during your cycle and with certainty if you became pregnant by in-vitro fertilization, for example.
The average gestation for humans is about 266 days from conception, so you can add 266 days to your conception date to get your approximate due date .
But here’s the catch — remember how we discussed that the egg could live for 24 hours and sperm can live up to five days? That means you can’t calculate your conception date as the day you had unprotected sex because you may have conceived several days after that.
Various Ways You Can Calculate Your Due Date
According to the American College of Obstetricians and Gynecologists (ACOG), the most accurate way to determine your due date is by ultrasound in the first trimester of pregnancy.
You can also use several due date formulas to calculate an estimated due date until you can get an accurate date by ultrasound.
- Using Naegele’s rule
Naegele’s rule is a standard formula to use if your menstrual cycle is a regular 28-day cycle with ovulation occurring on about day 14.
- First, determine the first day of your last menstrual period (LMP).
- Then, subtract three calendar months from that date.
- Lastly, add one year and seven days to that date.
For example, if your LMP was November 1, 2021, you would subtract three months to August 1, 2021. Then add one year and seven days for a due date of August 8, 2022.
- Using Mittendorf-Williams rule
The Mittendorf-Williams rule is considered highly accurate because it takes more information into account to calculate your due date. There are online calculators that allow you to answer various questions, and the more information you input, the more accurate your estimated due date will be.
A 1990 study by researchers Robert Mittendorf and Michelle Williams indicated that a first pregnancy lasts about 288 days from your LMP, but subsequent pregnancies last about 283 days from your LMP.
- First, determine the first day of your last menstrual period.
- Next, subtract three calendar months from that date.
- Then, add 15 days and one year to that date if it’s your first pregnancy, or add ten days and one year if it’s not your first pregnancy.
For example, if your LMP was November 1, 2021, subtract three months to August 1, 2021. If it’s your first pregnancy, add 15 days and one year to get a due date of August 16, 2022. If it’s not your first pregnancy, add ten days and one year to get a due date of August 11, 2022.
- Parikh’s rule
If your cycle is irregular, it may be helpful to use Parikh’s rule to determine a due date.
- First, determine the first day of your last menstrual period.
- Next, add nine months.
- Then, subtract 21 days.
- Lastly, add the average length of previous cycles in days.
For example, if the average length of your last 12 menstrual cycles is 30 days, and your LMP was November 1, 2021, you would add nine months to get to August 1, 2022. Then you would subtract 21 days back to July 7, 2022, and add 30 days to get a due date of about August 7, 2022.
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Are you struggling to make a decision about an unexpected pregnancy? Pregnancy Care Clinic offers no-cost medical services that empower you with the information you need to make a confident and informed decision.
 Knudtson, J., & McLaughlin, J. E. (2021, November 17). Menstrual cycle – women’s health issues. Merck Manuals Consumer Version. Retrieved from https://www.merckmanuals.com/home/women-s-health-issues/biology-of-the-female-reproductive-system/menstrual-cycle
 Ovulation, Conception & Getting Pregnant. Cleveland Clinic. (2019, February 19). Retrieved from https://my.clevelandclinic.org/health/articles/11585-pregnancy-ovulation-conception–getting-pregnant
 Hersh, E. (2018, October 26). Gestation: What it means in pregnancy. Healthline. Retrieved from https://www.healthline.com/health/pregnancy/what-is-gestation#calculate-a-due-date