Worried Mama!!

I had a checkup on Wednesday at 13+6 & he measured my fundal height & said it was measuring right at 16 so right on schedule but it took the dr a minute to be able to hear the heartbeat & then it wasn't for very long. He told me not to worry that he could hear the heartbeat through the placenta? I've just never had this happen before with my previous 2 pregnancies, so just looking for a little reassurance that everything is okay.