Calculating Conception Date: Ultrasound Measurements and Irregular Cycles

Calculation of the Conception Date

Hello, on November 16, the ultrasound at the first clinic showed 7 weeks and 5 days with a measurement of 1.6 cm, and the next day, November 17, another clinic showed 8 weeks with a measurement of 1.52 cm.
I would like to ask if the conception date is calculated by going back based on the ultrasound weeks? I have irregular menstrual cycles, and the doctor previously mentioned that I have symptoms of polycystic ovary syndrome, so I haven't been keeping track of my periods.
I started having unprotected intercourse with my boyfriend in the middle of this year, but on September 24, I had external ejaculation with my ex-boyfriend...
so I'm unsure whose child I might be pregnant with.
Thank you.

Yang, 20~29 year old female. Ask Date: 2016/12/04

Dr. Lü Lizheng reply Obstetrics and Gynecology

According to the ultrasound results from the first clinic on November 16, which indicated a gestational age of 7 weeks and 5 days with a measurement of 1.6 cm, the estimated conception date would be around October 8, counting five weeks and five days back from November 16.
On the other hand, the second clinic's ultrasound on November 17 showed a gestational age of 8 weeks with a measurement of 1.52 cm, suggesting a conception date around October 6, counting six weeks back from November 17.

Given the concern about potential measurement errors in the ultrasound, it is unlikely that the discrepancy would exceed three days.
However, considering that the sexual encounter on September 24 is 12 days prior to October 6, it is reasonable to conclude that the pregnancy is not from the previous boyfriend.

Sincerely, Dr.
Lu Li-Cheng, Taoyuan Hospital.

Reply Date: 2016/12/04

More Info

To determine the conception date and understand ultrasound measurements, especially in the context of irregular menstrual cycles and conditions like polycystic ovary syndrome (PCOS), we need to consider several factors.

Firstly, the ultrasound measurements you received are crucial in estimating gestational age (GA). The measurements taken during the ultrasound, such as the crown-rump length (CRL), are used to estimate how far along you are in your pregnancy. In your case, the first ultrasound indicated 7 weeks and 5 days (7w5d), while the second ultrasound showed 8 weeks (8w). These measurements can vary slightly between different clinics due to the equipment used and the technician's experience, but they generally provide a reliable estimate of gestational age.

To calculate the estimated conception date, we typically use the first day of your last menstrual period (LMP) as a reference point. However, since you mentioned having irregular cycles and a history of PCOS, this method may not be as straightforward. In a regular cycle, ovulation usually occurs about 14 days after the first day of your last period. If your cycles are irregular, ovulation can occur at different times, making it more challenging to pinpoint the exact conception date.

Given that your ultrasound measurements suggest you are around 7w5d to 8w, we can estimate the conception date by counting back approximately 2 weeks from the gestational age. For example, if your ultrasound indicates you are 8 weeks pregnant, this would suggest conception likely occurred around 6 weeks ago, or approximately 2 weeks after your last menstrual period.

Regarding your concern about the potential for conception from different partners, it is important to note that sperm can survive in the female reproductive tract for several days. If you had unprotected intercourse with your boyfriend after the encounter with your ex-boyfriend, it is possible to conceive from either partner, depending on the timing of ovulation.

In summary, while ultrasound measurements can provide a good estimate of gestational age, they should be interpreted with caution, especially in cases of irregular cycles. The estimated conception date can be calculated by subtracting approximately 2 weeks from the gestational age indicated by the ultrasound. However, given your irregular cycles and the potential for multiple partners, it may be challenging to determine the exact conception date without more detailed tracking of ovulation.

If you are concerned about the implications of your situation or need further clarification, it is advisable to consult with your healthcare provider. They can offer personalized guidance based on your medical history and current circumstances. Additionally, if you are experiencing any unusual symptoms or have concerns about your pregnancy, seeking medical advice is always a prudent step.

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