Estimating Conception Date and Paternity: A Guide for Women

I would like to know the approximate date of conception and who the partner is?

I have had regular menstrual cycles for the past few months, with a duration of 7 days and a cycle length of 32 to 33 days.
The dates of my menstrual periods are as follows: 2023/9/29 to 2023/10/5, 2023/10/31 to 2023/11/6, 2023/12/2 to 2023/12/8, 2024/1/4 to 2024/1/10, 2024/2/5 to 2024/2/11, and 2024/3/8 to 2024/3/14.
On March 28, I had sexual intercourse with Male A (without using a condom throughout, and the ejaculation occurred inside the condom, but I did not check for any exposure).
On April 4, I had unprotected intercourse with Male B, with ejaculation inside.
Currently, my period is 9 days late.
I took a pregnancy test on April 18, which showed two lines (though one line was lighter).
I would like to know approximately when conception occurred, whether the child is from Male A or Male B, or the probability of either.
If I know the gestational age or HCG levels, can I determine whether the child is from Male A or Male B? Is there a way to estimate the conception date or paternity in the early stages?

YY, 20~29 year old female. Ask Date: 2024/04/18

Dr. Cao Guotao reply Obstetrics and Gynecology

Hello, the chances are greater for B, but since the timing for both is close, we cannot say for certain who it is.
Wishing you good health.

Reply Date: 2024/04/18

More Info

Estimating conception dates and determining paternity can be complex, especially when multiple sexual encounters occur close together. Based on the information you've provided regarding your menstrual cycle and the dates of sexual activity, let's break down the situation.

Understanding Your Menstrual Cycle
You mentioned that your menstrual cycle is typically around 32 to 33 days long, with a duration of 7 days for your period. This means that ovulation likely occurs around 14 to 16 days before your next period starts. For example, if your last period started on March 8, 2024, you would expect ovulation to occur around March 22 to March 24, 2024.

Sexual Activity and Potential Conception Dates
You had sexual encounters on March 28 with A and on April 4 with B. Given that ovulation typically occurs about two weeks before your next period, the likelihood of conception from the March 28 encounter with A is lower, as it is outside the typical fertile window. However, the encounter on April 4 is closer to the ovulation window, especially if your cycle is consistent.

Pregnancy Test and HCG Levels
You mentioned that you took a pregnancy test on April 18, which showed two lines, indicating a positive result. The intensity of the lines can vary based on the concentration of hCG (human chorionic gonadotropin) in your urine, which increases as the pregnancy progresses. A faint line can indicate early pregnancy, and it is advisable to repeat the test a few days later or consult a healthcare provider for a blood test to measure hCG levels more accurately.

Estimating Conception Date
Based on your cycle, if you ovulated around March 22 to March 24, conception could have occurred from either sexual encounter, but it is more likely from the April 4 encounter with B, as it is closer to the ovulation date. However, sperm can survive in the female reproductive tract for up to five days, which means that sperm from the March 28 encounter could still be viable at the time of ovulation.

Determining Paternity
Determining the biological father based solely on the timing of sexual encounters can be challenging. If both encounters occurred within your fertile window, there is a possibility that either A or B could be the biological father. The most definitive way to establish paternity is through DNA testing after the baby is born.

Conclusion
In summary, while you can estimate that conception likely occurred around the time of your ovulation, determining the exact date and paternity based on the information provided is not definitive. The best course of action is to monitor your pregnancy closely, consult with a healthcare provider for further testing, and consider paternity testing after the birth for a conclusive answer.
If you have any further questions or concerns about your pregnancy or menstrual cycle, it's always best to consult with a healthcare professional who can provide personalized advice based on your medical history and current situation.

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