When Did Conception Occur? Pregnancy Timing and Ultrasound Results

When does conception occur?

Hello doctor, I do not have a record of my menstrual cycle, so I am unsure of the exact date of my last period.
On March 22, I had an ultrasound examination in the obstetrics and gynecology department, and the fetus was measured at 7 weeks and 5 days with a clear heartbeat.
As of now, at 29 weeks, the fetus is growing steadily and developing within the normal range.
I had sexual intercourse with man A at the end of January to early February, and with man B on March 4.
Could it be that I conceived before February (with man A) or after March 4? Is it possible for the fetus to grow that quickly? Thank you.

Niming, 30~39 year old female. Ask Date: 2022/08/18

Dr. Zhang Kunmin reply Obstetrics and Gynecology

Hello, based on your description, it usually doesn't happen this quickly.
Thank you! Wishing you peace and health.
Dr.
Chang

Reply Date: 2022/08/18

More Info

Determining the exact timing of conception can be quite complex, especially when there are multiple sexual encounters around the time of ovulation. In your case, you mentioned that you do not have a record of your last menstrual period (LMP), which makes it more challenging to pinpoint the exact date of conception. However, we can analyze the information you provided to help clarify your situation.

From the ultrasound conducted on March 22, which indicated that the fetus was 7 weeks and 5 days old, we can estimate that conception likely occurred around 7 weeks prior to that date. This would place the estimated conception date around early February. Given that the ultrasound is a reliable method for dating a pregnancy, it suggests that the fetus is developing at a normal rate for its gestational age.

You mentioned having sexual encounters with two different partners: one in late January to early February (A) and another on March 4 (B). Since the ultrasound indicates that the fetus is developing according to the timeline consistent with conception in early February, it is more likely that conception occurred during the encounter with partner A rather than after the encounter with partner B. Sperm can survive in the female reproductive tract for up to five days, so if you ovulated shortly after your encounter with partner A, it is plausible that conception occurred from that encounter.

It's important to note that the growth rate of the fetus observed in ultrasounds is typically consistent with the gestational age. If the fetus is growing normally and the ultrasound measurements align with the expected growth patterns, this is a positive sign that the pregnancy is progressing well. The fact that you have a clear heartbeat at 7 weeks and that the fetus continues to grow steadily at 29 weeks is reassuring.

In terms of the question of whether a fetus can grow rapidly enough to suggest a later conception date, it is generally unlikely. Fetal growth is relatively consistent and follows established growth curves. If the ultrasound measurements indicate that the fetus is within the normal range for its gestational age, it is unlikely that the fetus is developing at an accelerated rate that would suggest a later conception date.

In summary, based on the ultrasound findings and the timing of your sexual encounters, it is reasonable to conclude that conception likely occurred in early February with partner A. The growth of the fetus appears to be normal, and there is no indication of accelerated growth that would suggest a later conception date. If you have further concerns or questions about your pregnancy, it is always best to consult with your healthcare provider, who can provide personalized advice and support based on your specific circumstances.

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