pregnancy and sonogram question

hi all.. I'm very confused😔is it possible to be pregnant and baby not be seen on 17 week ultrasound?? 
hi all!! I have a complicated situation and need advice/help as I have NOO experience with ultrasounds or pregnancy😢😢 .. I missed my period last week and took a test- it was positive. Well I haven't had sex in more than 4 months so I called my obgyn yesterday and she got me an ultrasound today to see what's going on.. So I got there and she had only ordered for a pelvic ultrasound today. So I explained to the the sonographer that if I'm pregnant it would be like 17 or 18 weeks--- well we only did the transvag ultrasound today (internal and lower stomach- below my belly button)  and she said that she didn't see anything that looked like I am pregnant and that my uterus did not look enlarged, but that my obgyn would follow up... Is there any way that it would have gone unnoticed at ~17 weeks pregnant?? Thanks!Â