I have a 31 day cycle,it always comes on the same day every month .my due date is may,23,2013. My last period was August 17th 2012. I had a ultrasound on devember 17th 2012 that said I was 16 weeks and 4 days .When could I have conceived and was it a week before my period ?

Mrs2013-)/And I forgot to add could my conception date be September 9 or 10 th of 2012 ,or on CD24/25 on a 31 day cycle, which would only give me a 7-8 day lutreal phase ,which would be a week before my period was due. My cycle is never late either ,I got my BFP on the same day my period was suppose to start.

Was it a week before your period? Not likely, 16 weeks 4 days back from the ultrasound = Aug 23.
Then you ask, was it Sept 9 or 10? Not likely. That's 23/24 days after your LMP, and 17/18 days later than the doctor said.

Yes, you can get pregnant a week before your period, you can get pregnant any time during the month. It all depends on when you ovulate, and unless you were taking your basal temperature for 6 months and tracking ovulation, you won't know for sure when you ovulated.

Is this a matter of paternity? Is that why you want to know when you conceived? If so, the only answer is a DNA test after the baby is born. Ultrasounds are a guess, they're not accurate when determining conception dates.

Mrs2013) yes it is about paternity , I don't know when I ovulated , not the slightest clue ,the internet keeps saying if I have normal cycles , (which I do , I always start on the 17th) , I would ovulate on CD 14-18 - so the 9th would be CD23-24- of my 31 day cycle, then it says my lutreal phase must be 10-16 days long to sustain a pregnancy ,which would make my lutreal phase only 7-8 days long ,so Is it still possible to get a positive test 7 days after I had sex with the other person , and I took 2 weeks off the ultrasound done on December 17th and got September 6th because I know your not supposed to count the first 2 weeks because conception hasn't even occurred yet and the doctor confirmed my pregnancy and said I was 4 weeks which is really 2 weeks but could I have been actually 1 week fetus age , so 3 weeks gestational age and miss my period and have a positive test also , or is a week enough time for HGC to develop to confirm the pregnancy and enough time for the baby to be fertilized and implanted or did I conceive before that ,by like by a week

By checking my gestation wheel, the same one the doctors use, it appears you conceived on or around August 30.

With that said, since paternity is in question, it is in your best interests to have a paternity test done so there areno legal implications in the future.

Mrs2013) thank you so much and yes that was in my plans as well si to ease my mind a little its not possible to be 3 weeks pregnant with a missed period and 3 positive tests, right

It is possible. While most tests are not necessarily accurate until approximately 14 days late, early results do happen.

I have known of women how get a positive test before they are even late. Once a test is positive, you are pregnant. While false positives happen, they are quite rare.

Mrs2013) wow ,we learn something new everyday thanks for your feed back , and god bless

Just a friendly FYI. It's not necessary to type your username in your response.

As I said earlier, getting a DNA test is not only for peace of mind, but for future legal reasons.

I'm one of the women that always test early. With all three of my pregnancies I tested when I was only a few days late and had an accurate positive test. I'm not the norm, but it does happen. Most women get a more accurate result when they're at least 2 weeks late, and some don't get an accurate result until well into their pregnancy. All women are different, and every pregnancy is different. With pregnancy it's always safe to expect the unexpected. ;)