“When am I due?” It’s the first question women often ask when they find out they’re pregnant. But if you’re pregnancy is unexpected, you are more likely to ask, “When did this happen?” or “When did I conceive?”

There are many reasons you may want to know your conception date. You might need to pinpoint the possible father, examine your pregnancy options, or access prenatal care. Many people aren’t aware that all pregnant women need prenatal care, even if they haven’t decided if they will continue the pregnancy. Prenatal care simply means caring for women who are pregnant, and it helps them to examine their options based on each unique situation.

The best way to figure out when you conceived is by ultrasound. In this article, we’ll discuss the process of conception, various ways to calculate your conception date, and how we determine when you conceived based on ultrasound.

What Is Conception?

Many events need to come together just right for conception to occur [1]. After a woman’s ovary releases an egg (ovulation), it travels through the fallopian tube for about 30 hours. The released egg then rests for another 30 hours in a specific part of the fallopian tube, and this is where one sperm will penetrate the resting egg for fertilization. Sperm can live in a woman’s body up to five days, even though the egg can be fertilized only during a 12-24 hour window of time.

That moment of fertilization is your conception date.

The fertilized egg then begins its journey to the uterus. During this time, a woman’s body will produce more progesterone to thicken the uterine lining in preparation for the fertilized egg to implant. The egg will travel quickly through the fallopian tube to the uterus and burrow into the uterine lining.  This process is called implantation. The time between ovulation and implantation of a fertilized egg is about one to two weeks.

What Is the Difference Between Gestational Age and Conception?

When a healthcare provider tells you how far along you are in your pregnancy, you might think to yourself that it seems further along than you expected. That’s because a healthcare provider will give your gestational age.

Gestational Age vs. fetal age

Gestation Age is the time between your last menstrual period (LMP) and birth. Gestational age is the measurement used to tell you how far along you are in your pregnancy, and it’s measured in weeks from the first day of your last period [2] NOT CONCEPTION! This means that even though you weren’t pregnant yet, your last period counts as part of your pregnancy.

Healthcare providers use gestational age since ovulation occurs anywhere from day ten to day twenty in a twenty-eight-day cycle, which makes it almost impossible to pinpoint the exact conception date. Ultrasound has given us more accurate measurements, but tradition in Obstetrics has remained in use.

Fetal age is the age of the fetus, which is measured from the date of fertilization. The fetal age is usually two weeks behind the gestational age since conception typically occurs about two weeks from your last period. For example, if your gestational age is twelve weeks, your fetal age would be about ten weeks.

When Did I Conceive?

Unfortunately, there isn’t a way to know exactly when you conceived with certainty [3]. But, there are four ways that you can estimate a window of time, which can be helpful:

  1. Using your ovulation date
  2. Using dates of sexual intercourse
  3. Using the first day of of your last menstrual period (LMP)
  4. By ultrasound

When did I conceive based on ovulation date?

You might be very in-tune with your body and know when you ovulate each month. At about the middle of your cycle, you might experience ovulation pain, thinning cervical mucous, increased libido (sexual desire), and other symptoms.

If you know when you ovulated based upon your body’s signals, you can presume you conceived within 3 days of ovulation.

When did I conceive based on the dates of sexual intercourse?

You can keep track of your cycle with a period tracker app. If you do, it helps you to see patterns in your cycle to understand when you’re most likely to ovulate. It also allows you to keep track of days where you had sexual intercourse.

Knowing this information gives you a chance to see when intercourse coincided with ovulation, keeping in mind that sperm can live in your body for up to five days.

When did I conceive based on my last menstrual period (LMP)?

It’s nearly impossible to precisely know when you conceived based upon your LMP because the window of ovulation can vary considerably. Again, if you use a period tracker and have learned to recognize when you’re ovulating, you can get a more accurate conception date.

If not, you can still “guesstimate” that you conceived approximately 11-21 days from the first day of your LMP.

The gold standard of measuring when you conceived is by ultrasound.

When did I conceive based on ultrasound?

Ultrasounds measure gestational age most accurately in early pregnancy, which can be done as early as six weeks after your LMP, but it’s most accurate between eight and twelve weeks after your LMP.

The ultrasound technician measures the “crown to rump” (head to buttocks) length of the embryo/fetus and will give you the gestational age. To translate into fetal age and estimate when you conceived, take the gestational age and subtract two weeks. It should be noted that a Crown Rump measurements will have a degree of inaccuracy within a couple of days.

No-cost Ultrasounds

If you think you might be pregnant or have just discovered you’re unexpectedly pregnant, Pregnancy Care Clinic is here for you. We provide confidential, no-cost services by licensed medical professionals, including ultrasounds, to help you know how far along your pregnancy is and when you conceived. We take one step at a time with you and answer all of your questions without bias or judgment. Use our online Pregnancy Calculator to estimate your due date. Make an appointment today and be empowered to make an informed decision that’s best for your future.


[1] University of California San Francisco. (n.d.). Conception: How It Works. Retrieved on May 31, 2020 from https://www.ucsfhealth.org/education/conception-how-it-works

[2] Hersh, Erica. (2018, October 26). Pregnancy Lingo: What Does Gestation Mean? Retrieved on June 1, 2020 from https://www.healthline.com/health/pregnancy/what-is-gestation

[3] Gobinath, Aarthi. (2018, November 16). Conception Calendar: When Did I Conceive? Retrieved on November 10, 2019 from https://www.avawomen.com/avaworld/conception-calculator/