Calculating the Exact Date of Conception: A Guide for Women

Please estimate the accurate date of conception?

Hello Doctor: I am 37 years old, have never been pregnant, and am married.
My last menstrual period started on February 21, 2013 (my cycle is about 28 to 34 days, but I only remember that it came on November 23, 2012).
I had sexual intercourse on March 4 and March 14.
A pregnancy test on March 23 was positive (the second line was faint), and on that day, my blood HCG level was 154.2.
I had another blood test 48 hours later on March 25, and my HCG level was 368.
An ultrasound on March 28 did not show anything, but on April 1, a gestational sac measuring approximately 0.81 x 0.91 cm was observed.
Can I accurately estimate the date of conception as March 4 or March 14? This is important because it relates to the paternity of the child, and I am very distressed.
I have consulted many doctors, and their opinions vary; two suggested that conception occurred on March 14, as the gestational sac is about 1 cm in size at three weeks post-conception (but the male partner used a condom that day!).

1.
If conception occurred on March 14, would the HCG level on March 23 (nine days later) be 154.2, or would that level indicate approximately 14 days post-conception?
2.
Does using a condom significantly reduce the likelihood of conception on March 14? I sincerely request your response.
Thank you!

Chen Yuan, 30~39 year old female. Ask Date: 2013/04/02

Dr. Lü Lizheng reply Obstetrics and Gynecology

Based on the ultrasound on April 1, which showed a gestational sac measuring approximately 0.81 * 0.91 cm, it can be inferred that the pregnancy was at four weeks and six days.
Typically, the conception date is considered to be two weeks into the pregnancy, which means that from the conception date to the ultrasound, there is a difference of two weeks and six days.
Therefore, counting back from April 1 by two weeks and six days brings us to March 13.
Considering a possible three-day margin of error for the ultrasound, the conception date would likely be March 14.
The date of March 4 seems too far back to be plausible.
Additionally, a positive pregnancy test on March 23 (with a faint second line) supports the idea that conception occurred on March 14.
If conception had occurred on March 4, the pregnancy test on March 23 should have shown a clear positive result, rather than a faint line.
Finally, to be absolutely certain, one could wait until the pregnancy is further along to conduct a chromosomal test and compare the chromosomes of the father to confirm paternity.
Sincerely, Dr.
Li-Cheng Lu, Department of Obstetrics and Gynecology, Taoyuan.

Reply Date: 2013/04/02

More Info

To calculate the exact date of conception, especially in your case where there are multiple potential dates, we need to consider several factors, including your menstrual cycle, the timing of ovulation, and the nature of the sexual encounters.
1. Understanding the Menstrual Cycle: A typical menstrual cycle lasts about 28 days, but it can range from 21 to 35 days. Ovulation usually occurs about 14 days before the start of your next period. Given that your cycles are between 28 to 34 days, we can estimate your ovulation window. If your last menstrual period (LMP) started on February 21, 2013, and assuming a 28-day cycle, ovulation would likely occur around March 7. If your cycle is longer, ovulation could occur later, around March 14.

2. Timing of Sexual Intercourse: You mentioned having sexual intercourse on March 4 and March 14. If ovulation occurred around March 7, then intercourse on March 4 would be within the fertile window, as sperm can survive in the female reproductive tract for up to five days. Intercourse on March 14 would be after ovulation, making it less likely for conception to occur from that encounter.

3. HCG Levels and Pregnancy Testing: You reported a positive pregnancy test on March 23, with an HCG level of 154.2. HCG (human chorionic gonadotropin) levels typically double every 48 hours in early pregnancy. If conception occurred on March 4, the HCG level on March 23 would be consistent with approximately 19 days post-ovulation (or about 25 days post-LMP). If conception occurred on March 14, the HCG level would be consistent with about 9 days post-ovulation (or about 15 days post-LMP).
4. Interpreting HCG Levels: The HCG level of 154.2 on March 23 could be indicative of either conception date, but it is more aligned with the March 4 date, especially considering the doubling time of HCG. If you conceived on March 14, the HCG level would likely be lower than 154.2 at that point.

5. Impact of Condom Use: Using a condom significantly reduces the likelihood of conception, but it is not 100% effective. If the condom was used correctly and did not break, the chances of conception from the March 14 encounter would be very low. However, if there was any failure in the condom's use, it could potentially lead to conception.

In conclusion, based on the information provided, it is more likely that conception occurred around March 4, especially given the timing of ovulation and the HCG levels. However, if there are any concerns about the accuracy of this assessment or if you need further clarification, it is advisable to consult with a healthcare professional who can provide personalized guidance based on your medical history and circumstances.
Lastly, if you are feeling distressed about the situation, consider speaking with a counselor or therapist who can help you navigate your feelings and concerns regarding this matter.

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