My Obgyn Has An Old Ultrasound Machine And Hasn't Listen

my obgyn has an old ultrasound machine and hasn't listen for my babies heartbeat with a Doppler. I have had 3 ultrasound at 14,18,32 weeks and the baby is healthy and growing fine. with that being said at my 18 week check we seen a little girl on the ultrasound and it was clearly girl parts. when I went in at 32 weeks to make sure the baby was head down we noticed very distinct boy parts. I know that 18 week is a little early but the parts don't change from girl to boy. so is their a chance that the doctor just has seen a twin because he hasn't really look for them?