Hello.
Growth scans do exist, there are a whole load of fetal medicine consultants to prove this!
They look at bone length [the femur], the abdominal circumfrence, and the head measurements [depending on gestation] to get a feel for the way the baby is growing. If the measurements are in proportion, maintaining velocity along a centile and so on.
None of this can give a true weight indication, but can give a best guess. They can be inaccurate. The weight thing is often a bit of a red herring though. The wellbeing of the baby is determined by how sustained groth is, weather there are huge surges in growth that might point to a diabetes in the mother, if there is asymetric growth that might point to placental issues. Loads of clues as to how the pregnancy is functioning.
In your case, perhaps your consultant is worried about shoulder dystocia. It is very encouraging that you have birthed one large baby before, so perhaps the consultant can give you more info and what worried him/her. Babies do tend to get larger [not always] and previous baby over a certain weight is a known risk factor for a shoulder dystocia. A shoulder dystocia does sometimes throw out vauge clues in the latter stages of labour, but can also only be obvious when it is in progress. So it is classes as one of the obstetric emergencies. A growth scan in late pregnancy might help to give an indication of the size of the baby, and provide information for you all to make a decision with,
I am totally ignorant on MS so will not try to offer any comment on that.
Yes, midwives do have to support you in any care path you decide. They also need to try and inform you of the potential risks you take on by following any given path. If you take on making that decision yourself it is vital your decision is informed. I would urge you to discuss what specific concerns the consultant has and look for ways of informing the decision you make once you know what the risks you might be facing are.
Also talk to the midwife as much as possible, and get her take on it. As the lead clinician who books your home birth, she will be the one putting her name to the decision. She will be able to say how she feels about supporting your choices. If you can get the same one again, so much the better!
Good luck with whatever you decide!!