Can you listen for a fetal heartbeat with a doppler

I know that it is almost impossible to hear the fetal heartbeat with a stethoscope, but what about a doppler device? My daughter’s horse came out of a kill pen in May and she has shown no signs of pregnancy, other than not obviously going into season. The other day while hand walking my TB she was tied out while her stall was being cleaned. I noticed some obvious movement in her flank area and her abdomen feels tight (if that makes sense). I was able to catch a bit of it on video, the video is not the best because I had a restless tb on the lead line. I saw this several times in just about 5 to 10 minutes. I have never had any experience with this before so I am not sure what I am seeing. She came to us very underweight and was building muscle and weight nicely until the last month or so, she has just sort of stalled. Obviously now that I suspect she may be with foal she is being carefully fed accordingly and improving again. If you have a few moments to watch the video, I appreciate it. The obvious movement I see is around the 24 second mark right before she shakes her head.

https://youtu.be/w9UCB3B2ocM

I have had her stool checked and vet said it was good, no additional worming needed. I have talked to my vet, he has said there is no reason to haul her in and he will check her at his next visit. Obviously I would like to know either way so I thought maybe I could listen to the heart rate with the doppler. I will obviously have her examined when he comes out again. How far along are they usually if you can see fetal movement on the outside? Google is all over the place with this stuff. I know this is a lot of rambling questions, but I am a little bit freaked out. This is my first mare and I have only owned her since July. (She was purchased by my barn owner and I later bought her.) No access to a stud since May.

Thank you for your time.