Yes, most nutrition books will tell you that, but if you see your horses ribs. You're too late in increasing the feed.
I had to increase my mares feed in the fifth month. If the horse is too thin, it doesn't matter if books tell you that the mare don't need more nutrition until the 9th month. You have to look at the horse to know.
I rather have a mare that's a bit too fleshy when it's time, because sometimes, the mare stops eating before the foal drops, and if you got a mare that's thin, That could be a catastrophe for both the mare and the foal.
Some people are scared that the mare will have a foal that's too big if they feed the mare enough food to make her fleshy, but there's research done that tell us that mares don't have a problem delivering the foal even if the mare is fleshy and big.