Elective Ultrasound

Never thought I would be one of those people who pays for an ultrasound.. lol! But due to my work schedule and my dr being on vacation, I have to wait another 7 weeks to find out the sex. I thought I would find out at the end of the month. So my mom said she would pay for it—

but my question is: when is it TOO early to see anything? I will be 16 weeks Friday and my mom said we could do it this weekend. But I would hate for her to waste her money if the baby is ready to be seen!!