Calculating Your Conception Date: A Guide for Expecting Mothers

Q: Calculating Your Conception Date: A Guide for Expecting Mothers

Dear ABBY： Hello, on May 3rd, during an abdominal ultrasound, the healthcare provider measured the gestational sac and estimated it to be about five weeks. <br>On May 12th, another abdominal ultrasound showed a heartbeat, and she stated it was 6 weeks and 1 day. <br>Then on May 2

I would like to know the date of conception?

Hello Dr.
Lu, my menstrual cycle started on January 15, February 20, and March 21 this year.
I had a positive urine test on April 26, but the doctor said they couldn't see the yolk sac.
However, I saw a dark circle on the screen.
On April 29, during a transvaginal ultrasound, I also saw a dark circle, but the doctor still said they couldn't see anything.
On May 3, during an abdominal ultrasound, the healthcare provider measured the gestational sac and estimated it to be about five weeks.
On May 12, during another abdominal ultrasound, a heartbeat was detected, and it was said to be 6 weeks and 1 day.
Then on May 23, the abdominal ultrasound indicated 8 weeks and 2 days, and by June 10, it was noted as 10 weeks and 5 days.
Based on these three ultrasound measurements, my gestational age is estimated to be 5 to 9 days less than what would be expected from my last menstrual period (LMP).
Most recently, during the prenatal check-up on September 9, the doctor said I was 24 weeks and 2 days.
I would like to ask the doctor to help me estimate the possible date or dates of conception.
Thank you very much.

ABBY, 20~29 year old female. Ask Date: 2011/09/23

Dr. Lü Lizheng reply Obstetrics and Gynecology

Dear ABBY: Hello, on May 3rd, during an abdominal ultrasound, the healthcare provider measured the gestational sac and estimated it to be about five weeks.
On May 12th, another abdominal ultrasound showed a heartbeat, and she stated it was 6 weeks and 1 day.
Then on May 23rd, the ultrasound indicated 8 weeks and 2 days, and on June 10th, it was reported as 10 weeks and 5 days.
Based on the ultrasound results mentioned, the estimates from May 23rd and June 10th are likely the most accurate.
If the May 23rd ultrasound shows 8 weeks and 2 days, the conception date should be calculated as May 23rd minus 6 weeks and 2 days, which would be April 9th.
For the June 10th ultrasound showing 10 weeks and 5 days, the conception date would be June 10th minus 8 weeks and 5 days, which is April 10th.
Given the estimation from the ultrasound, there is a possible error of 3 days, so the conception date could range from April 6th to April 13th.
According to your previous menstrual records, the intervals between your periods have consistently exceeded 28 days, which is longer than the average female cycle.
Therefore, it is likely that ovulation occurred later, leading to discrepancies between the pregnancy weeks calculated from your menstrual cycle and those calculated from the ultrasound.
Current research suggests that the pregnancy weeks calculated from ultrasound are more reliable.
Wishing you peace and health.
Sincerely, Dr.
Lu Li-Cheng, Director of the Department of Obstetrics and Gynecology, Taoyuan Hospital, Ministry of Health and Welfare.

Reply Date: 2011/09/23

More Info

To calculate your conception date accurately, we first need to understand the relationship between your menstrual cycle, ovulation, and conception. Based on the information you've provided, your last menstrual periods (LMP) were on January 15, February 20, and March 21. This gives us a rough idea of your cycle length, which appears to be around 30 to 31 days.

Typically, ovulation occurs about 14 days before your next expected period. Therefore, if we assume your cycle is about 30 days long, ovulation would likely occur around the 16th day of your cycle. For example, if your last period started on March 21, ovulation would likely occur around April 5. This is the time when you are most fertile, and conception is most likely to occur if you have unprotected intercourse.

Given that you had a positive urine test on April 26, it suggests that conception likely occurred around the time of ovulation, which would be approximately April 5. However, sperm can survive in the female reproductive tract for up to five days, so if you had intercourse in the days leading up to ovulation, conception could have occurred a few days before April 5.

Now, regarding the ultrasound measurements you received during your pregnancy, it's common for there to be some variation in gestational age based on the size of the embryo or fetus. The ultrasound measurements can sometimes indicate a different gestational age than what is calculated based on your LMP. This discrepancy can be due to several factors, including the natural variability in fetal development and the timing of ovulation.

From your ultrasounds:
- On April 29, the ultrasound indicated a gestational age of about 5 weeks.

- On May 12, it showed 6 weeks and 1 day.

- By May 23, it was 8 weeks and 2 days.

- Finally, on June 10, it was 10 weeks and 5 days.

These measurements suggest that your embryo was developing at a rate consistent with a conception date around early April, likely within a few days of your estimated ovulation date. The fact that your ultrasound measurements are consistently a bit behind your LMP calculations (by 5 to 9 days) is not uncommon and can happen for various reasons, including the timing of ovulation and individual variations in fetal growth.

In summary, based on your menstrual cycle and the ultrasound data, it is likely that you conceived around April 5, give or take a few days. If you have any further questions or concerns about your pregnancy, it's always best to consult directly with your healthcare provider, who can give you personalized advice based on your specific situation. Congratulations on your pregnancy, and I wish you all the best as you prepare for your new arrival!

Similar Q&A

How to Calculate Your Due Date: A Guide for Expecting Mothers

How to calculate the due date?

Dr. Wang Jionglang reply Obstetrics and Gynecology
For women with a regular menstrual cycle, the estimated due date is typically calculated as: month + 9, day + 7 (for example, if the last menstrual period was on January 1, the estimated due date would be October 8). If the menstrual cycle is irregular, an early ultrasound can be...

Calculating Conception Date Based on Due Date: May 14th Insights

Hello, if the due date is May 14 of this year, the estimated conception date would be around August 21 of the previous year.

Dr. Huang Yiwen reply Obstetrics and Gynecology
Hello, Miss Xiao Xiao: If your menstrual cycle is regular and occurs once a month, then your estimated ovulation date would be around two to three days before or after August 21 of last year.

Calculating Conception Date Based on Due Date: A Guide

If the due date is September 20, the estimated conception date would be around December 27 of the previous year. Currently, you are at 19 weeks of gestation.

Dr. Cao Guotao reply Obstetrics and Gynecology
Hello, it should be around the end of December. Wishing you good health.

Calculating Conception Date Based on Due Date: A Guide for Expecting Mothers

Hello Doctor: My due date is currently November 4th. Can I calculate the conception date from this? Thank you!

Dr. Cao Guotao reply Obstetrics and Gynecology
Hello, it can only be roughly estimated to be around mid-February. Wishing you good health.