That's interesting Katy as I understood that placentitis was an infection that caused the placenta to harden or to become partially separated thereby depriving the foal of some of its essential nutriments, leading to either an early abortion or a week, compromised foal if the pregnancy does manage to go to term. Treatment, after culture of any discharge plus a u-sound, is to put the mare on anti-biotics for the rest of the pregnancy to try to 'cure' the infection and to reduce inflamation, together with something like regumate to try to keep the pregnancy going as long as possible for the (hopefully) foal's benefit.
I know that Robin had some discharge a while ago and that she started to develop an udder, both of which can be signs of placentitis, but I dont think that Robin can have been suffering from placentitis - perhaps she just had a minor infection for some reason - as your vet did not suggest continuing with the anti-b's and both she and your SIL are happy with Robin's progress now.
I was really interested to learn another aspect/possibility of placentitis from your SIL "
can speed up the development of the fetus". We are all here to learn so it is great to have another way of looking at what can be a problem with some mares.
I expect Robin is getting fluffy - my lot are well on the way towards getting dressed in their winter Yak fancy dress LOL!! And 300 days and your Birthday will be a great reason for a double celebration!!
Oh and keep the pictures coming.