Heartbeat getting slower?

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So I know that as pregnancy goes on and baby grows the heartbeat will start slowing down but when does that happen?

At 9w1d it was 167 and with my home Doppler it’s always been high 160’s to mid 170’s. Well today at 12w1d it was mid 150’s to low 160’s. Kind of a small change but I’m still slightly worried. Any insight?