Conception Timing: Key Questions for Early Pregnancy

I would like to know the timing of conception?

On May 20-30, I had a one-night stand with a friend, and during the same period, I also had intercourse with my boyfriend, which was unprotected.
On June 9, I took a pregnancy test, and the urine results showed one line dark and one line faint.
The doctor advised me to retest the following week on June 14.
I had a blood test, and I did not get my period on June 15, but I can't remember if it was a real period or if it was early pregnancy bleeding, as mentioned online.
From June 29 to July 3, I had intercourse with my boyfriend again, and on July 28, I took another pregnancy test, which indicated 4-5 weeks of pregnancy, but the ultrasound on August 7 only showed a gestational sac measuring 0.73 cm.
Throughout this process, I have been very worried and uncertain about whose child it is.
I would like to ask the doctor: 1.
If I do not know my menstrual cycle, can the ultrasound determine approximately when conception occurred? 2.
Is it possible to have conceived at the end of May but only discovered the pregnancy at the end of July? If so, I should be 8 weeks along in July, not 4-5 weeks.
3.
Is there a significant discrepancy between the ultrasound images and actual pregnancy? I would appreciate the doctor's assistance in answering these questions.

Iris, 30~39 year old female. Ask Date: 2023/11/19

Dr. Zhang Kunmin reply Obstetrics and Gynecology

Hello, based on your description, calculating the time of conception using ultrasound may have a certain degree of error and cannot be used to determine paternity.
It is recommended to use DNA testing as a reliable basis when necessary.
Thank you! Wishing you peace and health.
Dr.
Zhang

Reply Date: 2023/11/19

More Info

Understanding conception timing and early pregnancy can be quite complex, especially when there are multiple sexual encounters and uncertainties regarding menstrual cycles. Here are some key points to consider based on your questions:
1. Determining Conception Timing via Ultrasound: Ultrasound is a valuable tool in estimating gestational age. Typically, the first ultrasound performed in early pregnancy (around 6-8 weeks) can provide an accurate estimate of how far along you are based on the size of the embryo or gestational sac. If you are unsure of your last menstrual period (LMP), the ultrasound measurements can help determine the gestational age and, consequently, the approximate date of conception. However, it is important to note that while ultrasound can provide a good estimate, there can be variations in fetal growth rates, which can lead to discrepancies in dating.

2. Possibility of Late Discovery of Pregnancy: It is indeed possible to conceive at the end of May and not realize you are pregnant until late July. If you had a late period or experienced what you thought was menstruation, it could have been implantation bleeding or another type of early pregnancy bleeding, which can sometimes be mistaken for a menstrual period. If you were to have conceived at the end of May, by the time you took a pregnancy test in early July, you could be around 4-5 weeks pregnant, depending on when implantation occurred and how your body is responding.

3. Accuracy of Ultrasound Measurements: Generally, ultrasound measurements are quite reliable, especially in the first trimester. However, there can be some variability. Factors such as the position of the embryo, the skill of the technician, and the equipment used can all influence the accuracy of the measurements. It’s also important to remember that fetal growth can vary from one pregnancy to another, so while ultrasound can provide a good estimate, it is not infallible.

4. Understanding Early Pregnancy Symptoms: In early pregnancy, some women may experience light spotting or bleeding, which can be mistaken for a period. This is often due to implantation, where the fertilized egg attaches to the uterine lining. If you experience any unusual bleeding, cramping, or other symptoms, it’s essential to consult with your healthcare provider for further evaluation.

5. Confirming Paternity: If there are uncertainties regarding paternity due to multiple partners, the most definitive way to confirm paternity is through genetic testing after the baby is born. While it can be a sensitive topic, open communication with your partners and healthcare provider can help navigate this situation.

6. Follow-Up Testing: Since you mentioned that your pregnancy tests showed varying results, it’s crucial to follow up with your healthcare provider. They may recommend a blood test to measure hCG levels, which can provide more definitive information about the pregnancy's viability and progression.

In summary, while ultrasound can provide a good estimate of gestational age and conception timing, it is essential to consider the context of your menstrual cycle and any potential early pregnancy symptoms. If there are ongoing concerns about the pregnancy or uncertainties regarding paternity, discussing these openly with your healthcare provider can help you find the best path forward.

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