I agree with @Heyside Luv that the fact it's a PGD tested embryo is putting the odds in your favour that it's not a Blighted Ovum again that's causing the spotting this time.
The fact it hasn't gotten any worse or there's no cramping/severe cramping with it is also a good sign IMO. It could also be a breakthrough bleed that happens around the time a period would be due. The only thing you can do is to have another BT done to check levels.
A scan would show that the embryo is in the right place and would also show the sac and fetal pole. I don't think a HB can really be seen until 6w normally or until the HCG reaches a certain level (over 11,000 I think???).
Am and praying all is well and it's just your emby snuggling further in