ultrasound reading wrong?

[mamade4]

My last period was from June 6th to June 9th. I got a negative pregnancy test on June 30th and on July 4th, but a positive one on July 7th.  My periods are regular and usually last 28-32 days. Today I went to the my ob and he did a quick ultrasound. He just measured the femur and the head. The head gave me a reading of 16 weeks and 6 days while the femur gave me a reading of 18weeks and 1 day. My MA is 16 weeks and 2 days.  If I would have gotten pregnant in May wouldnt the June 30th test come out positive instead of negative? And also, when using ultrasounds, do ob's use gestational age which then I would actually be about 14 weeks instead of 16 weeks? Right?
Help, Very confused

[abbynalacharlie]

Some pregnancy tests are **** and don't work.

[alaysha]

Early pregnancy dating is when you can tell the approximent date of conception and due date. After 13 weeks babies develop at their own rate and your not going to get an accurate u/s. At this time they are just making sure it gets all its vital organs. And no ultrasounds use LMP.