How To Determine The Date Of Conception?

Posted on Tue, 17 Nov 2015

Question: Hello my name is xxxxxxxxx and I am currently 24 weeks pregnant and I am due on February 15, 2016. I've been very emotional and terrified. I am very sure on who the father of my child may be but sometimes I just wonder. I am 20 years of age and my off and off boyfriend is 47 years of age and I hope that we're expecting a child together. According to doctors my LMP was on May 11, 2015 but I believe my menstrual cycle started on May 7, 2015. I got off of my birth contraceptive pills in the end of April. Me and my boyfriend had unprotected sex a few.times in May. These dates include May 15, 2015, May 16, 2015, May 17, 2015, and May 19, 2015. We broke up on May 20, 2015.I had two different partners in XXXXXXX I had one encounter on XXXXXXX 3rd, 2015 but it did not last long at all but I have heard of pre ejaculation and I had sex with another guy on XXXXXXX 8th, 2015 but he used protection. I was expecting my next menstrual cycle to start between XXXXXXX 7, 2015 and XXXXXXX 11, 2015 but it never came. I did not have any symptoms until XXXXXXX 14, 2015 my body temperature was very high and I had nausea without vomiting I took a pregnancy test and it came positive. I also went to planned parenthood to confirm the pregnancy and they predicted by the blood test they had given me I was five weeks along. I had my first ultrasound and according to the ultrasound I was already 9 weeks and 3 days on July 17, 2015. I was thinking about getting a paternity test done once the baby is born to be sure of paternity. I talked to my boyfriend about this and the two other men but what should I do in this particular situation.

Answered by Dr. Jacqueline Brown (34 minutes later)

Brief Answer:
You conceived in mid-late May consistent with dates and sonogram

Detailed Answer:
Hello and I hope I can help you today.

A first trimester ultrasound and hormone levels is one of the most accurate ways to date a pregnancy and estimate a conception date. Measurements of the fetus can calculate most likely when you conceived, which may be a different day than what you expected based on your last period, because women do not necessarily ovulate at exactly 14 days after their period starts. In addition you don't necessarily conceive the day you have sex, because the egg and sperm can take even 72 hours to find each other for fertilization.

So if your due date based on the ultrasound is February 15th, that means you conceived somewhere between the middle and end of May. Which means that your boyfriend is most likely the father. It is really not possible you conceived in XXXXXXX

So it is up to you if you want to pursue paternity testing, but according to the information you provided for me, it is extremely unlikely that your partners in XXXXXXX could have fathered this baby. If they did, your due date would be almost in March.

I hope I was able to answer your question today and that this information was reassuring. If you need anything further, please do not hesitate to contact me.

Best wishes, and good luck with the rest of the pregnancy, Dr. Brown

Above answer was peer-reviewed by : Dr. Vinay Bhardwaj