Estimating Conception Date: Pregnancy Test Results and Timing

Estimated Date of Conception

Hello Doctor, I am #196777.
To summarize, the pregnancy test result at the clinic on 10/10 was negative.
I returned for a follow-up on 10/17 to confirm the pregnancy and would like to ask further questions.

Regarding your response: 1) A pregnancy test can only yield accurate results if intercourse occurred more than 14 days prior.
The negative result on 10/10 means that the intercourse that took place 18 days earlier on 9/22 and 14 days earlier on 9/26 may not have resulted in a detectable pregnancy.
Given that I had sexual intercourse on 9/19, 9/24, 9/26, 9/28, and on 10/3 and 10/5, is it possible that conception occurred from the intercourse on 9/24 or 9/26, which would not show up on the pregnancy test on 10/10?
2) You mentioned that the estimated date of conception was around 9/24 or 9/26.
The obstetrician indicated that the conception date was 9/30.
If the conception date falls around 9/24 or 9/26, could there be a discrepancy of 4-6 days compared to the actual date of 9/30?

Da C, 30~39 year old female. Ask Date: 2023/05/24

Dr. Huang Yuande reply Obstetrics and Gynecology

1.
It is indeed possible that a pregnancy test on October 10 may not yield a positive result, as most home pregnancy tests can typically detect pregnancy about 2 to 3 weeks after conception.

2.
This cannot be confirmed with 100% certainty; generally, the measurement of the embryo's crown-rump length via ultrasound has an estimated due date error of about three days.
Therefore, when calculating back from the date of intercourse, the margin of error is even greater.

Reply Date: 2023/05/24

More Info

Estimating the conception date and understanding the timing of pregnancy tests can be quite complex, especially when considering individual variations in menstrual cycles and ovulation. Based on the information provided, let's break down the key points regarding conception dates, pregnancy testing, and the implications of ultrasound findings.

Understanding Conception Dates
Conception typically occurs around the time of ovulation, which is usually about 14 days before the start of your next menstrual period. For women with a regular cycle, this can be easier to predict. However, for those with irregular cycles, pinpointing ovulation can be more challenging. In your case, if your last menstrual period (LMP) was on September 10 and you had sexual intercourse on September 19, 24, 26, and 28, it is plausible that conception could have occurred around September 24 or 26, especially if you ovulated shortly after these dates.

Timing of Pregnancy Tests
Pregnancy tests work by detecting the hormone human chorionic gonadotropin (hCG) in your urine, which is produced shortly after a fertilized egg attaches to the uterine lining. Generally, it is recommended to wait until at least the first day of a missed period to take a home pregnancy test for the most accurate results. Testing too early, such as on October 10, when your last period was expected around October 10, may yield a false negative result if hCG levels are not yet high enough to detect.

In your situation, since you had a negative test on October 10 but confirmed pregnancy on October 17, it is likely that the hCG levels were still rising and hadn't reached detectable levels at the time of your first test. This aligns with the common medical advice that testing should ideally occur at least one week after a missed period for reliable results.

Discrepancies in Estimated Conception Dates
The discrepancy between your estimated conception dates (September 24 or 26) and the date provided by your healthcare provider (September 30) can be attributed to several factors. Firstly, sperm can survive in the female reproductive tract for up to five days, which means that if you had intercourse on September 24, sperm could still be viable for fertilization when ovulation occurs a few days later. Additionally, the timing of ovulation can vary from cycle to cycle, even for the same individual, leading to variations in estimated conception dates.

Ultrasound measurements can also provide insights into gestational age, but they are not always perfectly aligned with the estimated conception date. Ultrasounds are typically more accurate in the first trimester, but factors such as the size of the embryo and the timing of the ultrasound can lead to variations in gestational age estimates.

Conclusion
In summary, based on your menstrual cycle and the timing of sexual intercourse, it is reasonable to estimate that conception likely occurred around September 24 or 26. The negative pregnancy test on October 10 may have been due to testing too early, and the confirmation of pregnancy on October 17 aligns with the expected rise in hCG levels. Discrepancies in estimated conception dates can occur due to variations in ovulation timing and the natural variability of sperm survival. It is always advisable to follow up with your healthcare provider for personalized guidance and to monitor the progression of your pregnancy through regular check-ups and ultrasounds.

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