Heart rate and gender

Went to one of my appointments recently and baby's heartbeat is now 143, no big deal right? My soon to be MIL is disappointed now because "it's going to be a boy". This is after she was soo sure it was a girl after my first appointment because the doppler registered the heartbeat at 160. Why would you put so much stock in an old wives tale? She's had 5 pregnancies and had six children (last pregnancy was with twins) you'd think she'd be aware of how much the heart rate changes throughout the pregnancy. And she's acting mopey as if it's not her damn grandchild regardless of gender! It's ridiculous and I just needed to vent about it.