Determining Your Conception Date: A Guide for Expecting Mothers

I would like to inquire about the days when conception occurs?

The last menstrual period started on July 27, lasting approximately 7-8 days.
The average menstrual cycle is 28-31 days.
A pregnancy test using Clearblue on September 1 indicated a pregnancy of 3 weeks and a few days.
An ultrasound on September 4 showed measurements of D1* 0.60 cm and D2* 0.49 cm, but the gestational age could not be determined.
On September 18, the ultrasound indicated 7 weeks and 6 days (CRL 1.31 cm).
On October 20, the ultrasound showed 12 weeks and 3 days.
On December 14, the ultrasound indicated a gestational age (GA) of 20 weeks and 2 days (AUA).
Unprotected intercourse occurred from August 5 to August 12 (with ejaculation inside), and there were a few days of safe sex from August 22 to August 29 (except for external ejaculation on August 24).
I would like to know approximately when conception occurred.
Why is there a two-day discrepancy between the gestational age (GA) measured by ultrasound (AUA) and the estimated due date (EDD)?

Vicky, 30~39 year old female. Ask Date: 2018/12/17

Dr. Huang Jianzhong reply Obstetrics and Gynecology

The last menstrual period started on July 27, lasting approximately 7-8 days.
The average menstrual cycle is 28-31 days.
A pregnancy test using Clearblue on September 1 indicated a pregnancy of 3 weeks and a few days.
An ultrasound on September 4 showed measurements of D1* 0.60 cm and D2* 0.49 cm, but the gestational age was not visible.
On September 18, the ultrasound indicated 7 weeks and 6 days (CRL 1.31 cm).
On October 20, the ultrasound showed 12 weeks and 3 days.
An ultrasound on December 14 indicated a gestational age (GA) of 20 weeks and 2 days (AUA).
Unprotected intercourse with ejaculation occurred between August 5 and August 12, and there were a few days of safe sex from August 22 to August 29 (except for external ejaculation on August 24).

I would like to ask when the estimated conception date would be.
Also, why is there a two-day discrepancy between the GA (AUA) from the ultrasound and the GA (EDD) date?
Answer: The estimated conception date is likely around August 5 to August 12, and the discrepancy in gestational age is merely an estimation.

Reply Date: 2018/12/17

More Info

Determining your conception date can be a bit complex, especially when considering various factors such as your menstrual cycle, ovulation timing, and the results of ultrasounds. Based on the information you've provided, let's break it down step by step.

1. Menstrual Cycle and Ovulation: You mentioned that your last menstrual period (LMP) started on July 27 and lasted for about 7-8 days. With an average cycle length of 28-31 days, ovulation typically occurs around 14 days after the start of your last period. This would place your ovulation window around August 10 to August 13.
2. Conception Timing: Since you had unprotected intercourse from August 5 to August 12, it is likely that conception occurred during this window, especially if you had intercourse close to your ovulation date. Given that sperm can survive in the female reproductive tract for up to five days, it is plausible that conception could have occurred from August 10 to August 12.

3. Ultrasound Measurements: The ultrasounds you had provide additional context. On September 4, the ultrasound showed a gestational sac measuring 0.60 cm and 0.49 cm, which is quite early in the pregnancy, and it’s common not to see a clear gestational age at this point. By September 18, the ultrasound indicated you were 7 weeks and 6 days pregnant, which aligns with the conception date we estimated.
4. Gestational Age (GA) and Estimated Due Date (EDD): The discrepancy you noted between GA (AUA) and GA (EDD) could be due to the method of calculation used by the ultrasound technician. The gestational age is typically calculated based on the size of the embryo or fetus, while the estimated due date is often calculated from the first day of your last menstrual period. This can sometimes lead to variations in the dates, especially if the embryo is measuring slightly larger or smaller than expected for the gestational age.

5. Conclusion: Based on your LMP and the timing of your ovulation, it is reasonable to conclude that your conception likely occurred between August 10 and August 12. The ultrasound measurements seem to support this timeline, as they align with the expected growth of the embryo during that period.

If you have further questions or concerns about your pregnancy, it is always best to consult with your healthcare provider. They can provide personalized insights based on your medical history and the specifics of your pregnancy. Additionally, keeping track of your menstrual cycle and any changes can help you better understand your reproductive health moving forward.

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