Determining the Most Likely Conception Date: A Guide for Women

Date of conception

Hello Dr.
Tsao, my menstrual cycle in October was from October 9 to 12, and my previous cycles were about 28 days, occasionally arriving earlier or later.
I took emergency contraceptive pills for about 15 days at the end of August and beginning of September (for a trip).
My September cycle was from September 15 to 19.
I had unprotected intercourse on October 13, 17, 21, and between October 23 or 24, and October 30 or 31, with external ejaculation only on October 13 and 21.
The app predicted my period would start on November 4, but it did not come.
I took home pregnancy tests on November 6-8, which all showed one line.
I initially thought hyperthyroidism might be affecting my hormones, so I took another test on November 17 and got two lines.
On November 22, I went to the clinic for an abdominal and vaginal ultrasound, but no gestational sac was detected.
I plan to go for another scan on November 29.
Could you please tell me which date is most likely for conception? Thank you! 🙏

Y, 30~39 year old female. Ask Date: 2021/11/22

Dr. Cao Guotao reply Obstetrics and Gynecology

Hello, it could be on October 13th or 17th.
Wishing you good health.

Reply Date: 2021/11/22

More Info

To determine the most likely conception date, it is essential to understand the menstrual cycle and how ovulation works. Based on the information provided, your menstrual cycle is approximately 28 days, which is considered regular. Ovulation typically occurs about 14 days before the start of your next period. Therefore, if your last menstrual period started on October 9 and lasted until October 12, your next expected period would be around November 6, assuming a 28-day cycle.

Given this timeline, ovulation would likely occur around October 23. This is the day when you are most fertile, and the days leading up to ovulation are also considered fertile days. Sperm can live inside the female reproductive tract for up to five days, so any unprotected intercourse in the days leading up to ovulation can result in conception.
From the dates you provided, you had unprotected intercourse on October 13, 21, and 23. The most likely date for conception would be October 23, as it is the closest to your ovulation date. The intercourse on October 21 could also potentially lead to conception, but it is less likely than the intercourse on the day of ovulation itself.

Regarding the pregnancy tests, it is important to note that testing too early can lead to false negatives. Home pregnancy tests are most accurate when taken after a missed period. Since your period was expected on November 4 and you tested on November 6-8 with negative results, it is possible that the hormone levels were not high enough to detect pregnancy at that time. The fact that you received a positive result on November 17 suggests that you were indeed pregnant, but it may have been too early to detect it with the earlier tests.

The ultrasound results indicating no visible gestational sac could be due to several factors, including the timing of the ultrasound. If you are indeed pregnant, it may be too early to see the sac, especially if your ovulation date was later than expected or if implantation occurred later. It is advisable to follow up with your healthcare provider for further evaluation, as they may recommend another ultrasound after a week or so to check for the development of the pregnancy.

In summary, the most likely conception date is October 23, based on your ovulation cycle. If you have concerns about your pregnancy or the results of your ultrasound, it is crucial to communicate with your healthcare provider for personalized advice and care. They can provide you with the best guidance based on your specific situation and health history.

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