I would trust the ultrasound tech, that's just a wives tale, even if it's right for some people. Congratulations and good luck!
We did it before i was pregnant, we did the ring test, basically same thing it said 3 boys, were expecting our first son Thursday :)
That test your mum did is probably some old wives tale thing, the ultrasound is a trained person actually looking at the parts of ur babies body! I think id rather trust them, though im sure there have been mistakes made in the past so u never know! Just keep reciepts for anything u buy just incase lol