Worry Wart

Elizabeth
After years of fertility issues, my wife is pregnant with our first child after an <a href="https://glowing.com/glow-fertility-program">IUI</a> with donor sperm! We are 6 weeks 4 days along and it has been nothing but stress since the day we found out. We started with slow rising HCG and now we are praying for a heartbeat. This Tuesday will be our third ultrasound. The first one was to make sure the gestational sac was in the uterus, then last week there was a yolk sac and they got a CRL measurement. 
They gave us a big scare though and said they wish they had seen the heartbeat last week (5 weeks 6 days) but they didn't so we have been anxiously waiting for this Tuesday's appointment. Isn't there still time for a heartbeat?? I know at a fertility clinic they have seen so much loss so they have to be realistic, but it's been such a roller coaster. 
Today my wife's areolas have not been as big and we have been in a funk all day thinking something is wrong. Any advice to put our minds at ease would be greatly appreciated!