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Calculating Conception / Due Dates
See the Pregnancy Calculator for more information.
Q. On November 3rd I had an ultrasound and the results stated that I was 12 weeks and 5 days pregnant. From that can you help me determine my due date?
A. Ultrasound development results are measured by an average (the fetus is 2 1/2" long at 12 weeks, etc.). Because your baby may be a little ahead or behind, it can throw off your due date, as compared to the period-calculated date, by a week or two.
If we are calculating strictly by your ultrasound results, your due date should be May 12th.
Hell, there are a lot of factors that can throw off your due date. Such as…when the baby feels like coming out. No one I know has ever had her baby on the actual due date. I think it's best to plan roughly within that month, give or take a few days.
Q. My daughter just had her initial exam. The doctor said she was 11 weeks along and that her due date is August 12. The first day of her last period was on November 16. By most calculations that would make her due date August 23. By the doctor's due date, from everything I have been reading, the first day of her last period would have been on November 6, so how did the doctor arrive at the August 12 due date? Also, if the doctor's due date is correct, what is the conception date?
A. I think the doctor just made a mistake. Doctors are only human, after all. Maybe the doctor thought your daughter said, "November 6th" instead of "November 16th" as the first day of her last period.
The conception date should be about two weeks after the first day of the last period - around December 7th.
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Q. I had my period on the 5th of December and I had sex on the 14th with someone other that my boyfriend. He did not cum in me. On the 19th my boyfriend was home for 14 days and we had unprotected sex every day and he came in me several times. Today is the 5th of January and I did not start my period. I took a pregnancy test and it was positive. What are the chances that I am not pregnant by my boyfriend? When was the time I probably conceived?
A. Most women ovulate about 12-14 days after the first day of their period, making your most likely time of ovulation from the 17th to the 19th. Of course every woman is different and this can vary by a few days. There is a chance that the other guy could be the father if you ovulated on the earlier side and/or his sperm survived for a few days. The odds are more in favor of your boyfriend being the father– making his sperm deposit on the 19th. But, there really is no way to tell for sure without a paternity test after the baby is born.
Q. In August my fiancé and I broke up and I had sex with someone else on August 9. I started my period on August 19. I had another period on September 13. My fiancé and I worked things out and are back together and happily expecting a child. Is there any way this child could have been conceived in August? My doctor says I am 14 weeks pregnant with a due date of June 19.
-Alyssa, North Dakota
A. If you had sex with your fiancé in September, he is most likely the father. This would coincide with the June 19 due date. I know calculating conception dates vs. due dates can be confusing. OB/GYNs estimate your due date by seemingly tacking on an extra two weeks from the actual conception date–- making you more pregnant (week-wise) than you thought you were. See the "the pregnancy calculator" for more details.
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Q. I got pregnant in September. I cheated on my husband 9/10-9/11-9/21. My last period in August began 8/30. I bled again 9/20 during sex but only for 3 days. I had an ultrasound done 10/26 that told me I was 5wks 6 days. Is there any chance I could be pregnant from the other guy? Could I have become pregnant during my period? I took a pregnancy test 9/21 and it came back negative.
A. At this stage of the game, a sonogram is not a completely accurate gauge for judging your due date/conception date. It could be off by a week or two.
Either your ultrasound estimate is off by about two weeks and your husband is the father, or the embryo survived your 8/30 period (a rare, but possible occurrence) and the other guy is the father. Even if you did become pregnant by the other guy, a pregnancy test taken less than 10 days later would not show a positive result. It usually takes two to three weeks for a positive result to register on a pregnancy test.
In any case, your baby's paternity is unclear. You can discuss with your doctor your concerns about the ultrasound results and the period you've had that doesn't coincide with that estimate.
Q. I miscarried in my 5th week of pregnancy and then shortly thereafter found out I was pregnant again. I know that an estimated due date is normally derived from the first day of your last period, but I never had one. I would assume I am somewhere between 7 and 9 weeks along now and I am patiently waiting for my first appointment, but I was wondering if you know of a general rule for seeing how far I am that doesn't include the last period. Thanks!!
A. Hmmmm. That's a tough one. I would guess at counting the date of your miscarriage as the first day of your last period. In later months, a sonogram will give you a more accurate estimate. For now, if you really want to pinpoint your due date, ask your doctor if she/he has a formula for estimating it.
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Q. I'm not sure if I'm telling the right man he is my baby's father. We had sex on April 6,2003 and April 12, 2003. But on July 23, when I had my ultrasound, it said I was 17 weeks pregnant. Here is where I'm confused- When the pregnancy is dated from LMP (last missed period), conception date is already considered two weeks pregnant. Are ultrasounds done the same way? I'm asking because if you count back 17 weeks from July 23, It puts you at about March 26. During that time I was with another man that my alleged baby's father doesn't know about. If the two weeks are included, then it works out. That two weeks could mean the difference in my being right or suffering a whole lot of embarrassment. Please help! Thank You.
A. I agree- it is very confusing trying to figure conception dates vs. due dates.
When I started my pregnancy journal, the first chapter was titled "Week Four." I had just missed my period and discovered that I was pregnant. Therefore, I should have been two weeks pregnant. Right?
The reason I started with "week four" instead of "week two" is because when doctors gauge your due date, they start counting from the first day of your last period. Of course there is no way you could be pregnant then because, chances are, you wouldn't be ovulating for another twelve to fourteen days.
But, if you want to keep up with doctor-speak, as far as what things to expect at the weekly milestones, it's best to stick with their schedule. To answer your question, the ultrasound technician should also be estimating your due date based on the same formula (involving your last period).
It's very likely (but not guaranteed) the first man you mentioned is the father of the baby. The only way the March 26 man could be the father is if he had bionic sperm that survived your menstruation that should have happened on March 30.
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Q. My girlfriend had sex with me and another guy within the same 24-hour period. Now she is pregnant. Is it possible that the baby is his if she had sex with him two days after her last period?
A. Most women ovulate about 12-14 days after the first day of their period. This is when they are most receptive to pregnancy. If your girlfriend menstruated for the average seven-day period, then she would be due to ovulate about five days later. Once sperm is deposited, it can sometimes live for two to three days, waiting around for the egg. Of course there is some variation from person to person, but it looks like the other man does have a chance of being the baby's father. Unless one of you is known to be sterile, the only way you can determine the father with certainty is to undergo genetic testing after the baby is born.
Q. I had a one night stand with a co-worker on June 12th. I was in a monogamous relationship prior to that with my boyfriend of seven months. I got my period from June 16-21st. The next month I discovered I was pregnant. I'm 25 weeks now, and want reassurance that it can't be from my one night stand since I had a full-blown period. Please Help!
A. It's highly unlikely you became pregnant on June 12th. It's rare that a newly implanted embryo can survive the shedding of the uterine lining.
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Q. I'm 32 weeks pregnant with my third child. I was seeing another man when I got pregnant. He never ejaculated in me, but we didn't use condoms either. How do I know if this is his child or my husband’s?
A. If you had sex with the other man about 30 weeks ago, he does have a chance of being the baby's father. If you didn't have relations with him around that time, the baby is most likely your husband's.
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