Conception Dates Based on Ultrasound and Menstrual Cycle

Possible date of conception

Hello doctor, I would like to ask about the ultrasound examination.
The first day of the last menstrual period was on January 15, and the estimated due date is October 22.
The ultrasound on March 13 showed 8 weeks and 1 day with a crown-rump length (CRL) of 1.69 cm.
The ultrasound on April 3 showed 11 weeks and 5 days with a CRL of 4.94 cm.
The ultrasound on April 13 showed 12 weeks and 4 days with a CRL of 6.19 cm.
Is it possible that conception occurred from intercourse on the first day of the last menstrual period (January 15)? Thank you.

gǒu gǒu, 20~29 year old female. Ask Date: 2022/01/14

Dr. Zhang Kunmin reply Obstetrics and Gynecology

Hello, the error in calculating the gestational age based on the weeks of pregnancy can be significant.
For individuals who engage in frequent sexual activity, it is not advisable to use this method for calculation, as it often leads to unnecessary disputes.
Thank you! Wishing you peace and health, Dr.
Zhang.

Reply Date: 2022/01/14

More Info

To understand conception dates based on ultrasound and menstrual cycles, it's essential to grasp how pregnancy dating works. Pregnancy is typically calculated from the first day of the last menstrual period (LMP). This method assumes that ovulation and conception occur approximately two weeks after the LMP in a standard 28-day cycle. However, variations in cycle length and ovulation timing can affect this estimate.

In your case, the first day of your last menstrual period was January 15. If we consider a typical 28-day cycle, ovulation would likely occur around January 29, making this the most probable date for conception. However, if your cycle is longer or shorter, this date could shift accordingly.
The ultrasound findings you provided indicate the following:
- On March 13, the ultrasound showed 8 weeks and 1 day of gestation, with a crown-rump length (CRL) of 1.69 cm.

- On April 3, the ultrasound indicated 11 weeks and 5 days of gestation, with a CRL of 4.94 cm.

- On April 13, the ultrasound showed 12 weeks and 4 days of gestation, with a CRL of 6.19 cm.

These ultrasound measurements suggest that the fetus is developing at a rate consistent with the gestational age calculated from the LMP. The gestational age (GA) based on ultrasound is often considered more accurate than LMP, especially if there are discrepancies in cycle length or ovulation timing.

Given that the ultrasound on March 13 indicated 8 weeks and 1 day, this aligns well with a conception date around January 29. If conception occurred shortly after your last menstrual period, it would be plausible, but it is more likely that conception happened around the time of ovulation, which is typically two weeks after the LMP.

Regarding your question about whether conception could have occurred on the first day of your last menstrual period (January 15), it is highly unlikely. Sperm can survive in the female reproductive tract for up to five days, but conception typically does not happen until ovulation occurs. Therefore, unless you had an unusually short cycle or ovulated earlier than expected, conception on the first day of your period is improbable.

In summary, based on the information provided, it is most likely that conception occurred around January 29, following the typical ovulation pattern. The ultrasound measurements support this timeline, indicating that the fetus is developing as expected for the gestational age. If you have any further questions or concerns about your pregnancy, it is always best to consult with your healthcare provider for personalized advice and information.

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