Hi!
You feed a perfectly normal grass hay based diet; please DO NOT up the pellets. In all honesty, we haven't seen any difference whatsoever in births where a pregnancy was known or expected and in surprise births. The extra amounts are truly minute and already largely covered in a good diet and on good care.
What makes a big difference is if the normal diet and care is sub-standard - sadly that is very often the case with breeders, especially with for sale breeders and with people who aim for cheap pets and spending as little money and care as possible or in the case of rescues with new intakes from a bad background. In this case, extra alfalfa is the quickest way of making up lost ground. But if you are already ahead of the game, then there is no reason to throw things at your girl that are not needed.
The most important pregnancy preparation is to aim at your mother being as fit and healthy as can be and to keep her as normally active as possible; this will trickle down to any babies and makes a real difference in their overall survival chances. Don't feed more pellets which distract from her as much hay as possible and replace a bigger volume of hay with a comparatively smaller amount of mostly empty fillers - you are aiming at healthy babies of ideal weight and not at large babies that are at risk of getting stuck with potentially fatal consequences.
I hope that that makes sense?
Please take the time to carefully read our diet recommendations at the top of the pregnancy information link and keep in mind that it is the general diet that is the key and not the little tweaks you make in the last three weeks when the babies are showing and growing fast. The tweaks are not so much for the babies but to protect the mother's own bodily reserves during the stage when her most of what she is eating is diverted towards the pups just before giving birth and while she is nursing them in the first weeks after birth.
Pregnancy, Mother & Baby Care Guides