I borrowed my gf's sononline b. There's tons of YouTube vids on using it and a lot of ppl have reported being successful with it as early as 8 weeks! I started using it at 10 wks and it does recommend you not use it till 12 wks. But that isn't bc it causes issues, it's bc it can be very difficult to find and cause undue stress and worry. So just keep that in mind.
There have been a few nights where we gave up bc the baby moved and it took too long to find it. The thing is to not panic, usually we found the baby the next day.