i need help

[Markslove]

I had my 12 week dating scan a couple of weeks ago and now i am 17 week, but when i count back 17 week its the 31st of march, this really confuses me as my fiance wasnt home till the 3rd of april till the 23rd as he is in the army. I did a pregnancy test on the 26th april and it was negative and it didnt say positive untill the 5th of may how long does it take for a pregnancy to show on a test and is it possible that my dating scan is wrong?? or is it possible it could be a bigger baby? or would they have picked up on that??
Thanks x

Your pregnancy is dated from the first day of your last period. So, if you have a regular cycle of 28 days then you will generally ovulate about day 14 of your cycle. If you are dated at 17 weeks it is only 15 weeks since conception. Hope this helps

it's the random 2 weeks they add on to make it a nice round 40 week pregnancy - did my head in first time round but hey ho!

Pregnancy dates from 14 days from (likely) conception, so the blastocyst is already "two weeks old" when it comes into existence. Your scan indicates that you conceived around 14 April, although it could have been later depending on your usual cycle.

Once an egg is fertilised, it travels to the uterus for implantation, which takes 10-14 days.

Once implanted, the body gets the signal to produce hCG, the hormone that is detected in pregnancy tests. The levels of hCG rise steadily from implantation throughout the first trimester of pregnancy and reach a peak 60 to 80 days after the fertilized egg implants.

When you tested on 26 April, it either wasn't yet implanted, or had only just been so, and so the hormone levels were too low for the test to detect it.

Thanks for the replies has helped alot, i was due a perion on the 12th of april but that never came so it all makes sense what you've all said thanks.

Only 4% babies are born on their due date, ( I think) and at the end of the day, it's just a rough estimate of when you conceived. There is no way to be exact.
I'm sure I conceived before they think I did, so I have my edd at about 7 days before my actual one. Also, this is my first, and when I plotted the head, abdominal circumference on the graph, it is right on the brink (top end) of 'normal'. Which also makes me think they have it wrong. Gueass I'll find out soon though haha