That is what they used to say about babies. The slower the heartbeat the more likely it was a boy. I had one big lazy girl that I was so sure was a boy that I almost wrote our "boy" name in her baby book before she was born.
Our 8 week old puppies were just checked by the vet. He said all of the larger puppies had slow heartbeats, and the little guys were up to 20 beats a minute faster. Their weights are all between 1.9 lbs. and 2.3 lbs. so not that different.
IMO it would be very difficult to accurately predict sex of a puppy before birth.
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LaVail Yorkshire Terriers |