Measuring small, possible slow heartbeat outcomes

Hi all. Long time lurker, first time poster. We are finally pregnant after a ruptured ectopic pregnancy with tube removal 18 months ago. I went today for what should have been a 7 week, 1 day ultrasound. The baby was measuring only at 6 weeks 4 days. Also, the heartbeat looked slow to me but the doctor told me “they don’t measure the heartbeat this early.” Does that sound accurate? And has anyone had a baby measure behind at the first ultrasound and gone on to have a healthy pregnancy? I’ve had no symptoms but am absolutely sure of my dates so it’s all disheartening. Thanks in advance if you read through this!