How to Calculate Conception Date From Ultrasound

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

There are many important reasons you may want to know your conception date. You may need to pinpoint the possible father, examine your pregnancy options or access prenatal care. All pregnant women need prenatal care, even if they haven't decided if they will continue the pregnancy. Prenatal care helps women examine their options based on their unique situations.

We’ll talk about various ways to calculate your conception date, including how to calculate your conception date from an ultrasound.

How Does Conception Occur?

Many events need to come together just right for conception to occur [1]. After a woman’s ovary releases an egg (ovulation), the egg travels down the fallopian tube to make its 30-hour journey to the uterus. Sperm can live in a woman's body for several days, even though the egg can be fertilized only during a 12-24 hour window of time.

If the egg is fertilized in the fallopian tube, a woman’s body will produce more progesterone. This thickens 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. It’s the implantation date that’s considered the conception date rather than the date of fertilization.

Ways to Calculate Your Conception Date 

You can calculate your estimated conception date by your ovulation date, the date of your last menstrual period (LMP), or from an ultrasound [2].

Calculating by your ovulation date

Some women know the day they ovulate. If you know the day you ovulated, you can add nine days to give you an approximate conception date.

Calculating by the date of your LMP

If you know the date of your LMP, add twenty-three days to give you an approximate conception date. If your periods are irregular, this calculation may need to be verified by an ultrasound.

Calculating by ultrasound

The most accurate way to calculate your conception date is by ultrasound.Ultrasounds in the first twelve weeks of pregnancy are quite accurate in determining the age of a pregnancy.

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. 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] Nall, Rachel. (2018, August 30). All About Conception. Retrieved on November 10, 2019 from

[2] Gobinath, Aarthi. (2018, November 16). Conception Calendar: When Did I Conceive?Retrieved onNovember 10, 2019 from