How does heartbeat tell the gender?


Considering the heart rate is lower at say 6 weeks and rises before lowering, how can the old wives tail say under or over 140

Is a gender when you’re likely to catch it under 140 if you go at 6 weeks and it’ll likely be over 160 is you go at like 8 weeks? Just curious, i know wives tales are only 50/50, but just questioning the logic so many go by!