Rate of growth for an underweight snake?

I have an estimated 3 - 3.5 y.o. female bp that only ate twice in an 8 month period. She was kept at several different homes at this time due to moving/power outages/construction/etc. A lot of upheaval and traveling and new environments. At the beginning of August she ate again, and has had 3 consecutive feeds interrupted by a long over-due shed. Her appetite is great, she's looking better, more active, and seems to finally be settled into our long-term home. But she is definitely underweight (not enough girth, spine more visible than usual, etc). I lost my food scale in the move and I'll be purchasing another one today to keep track of her gains. I'm wondering, if she's eating a small rat each week, what is the expected gain from that? As in, how many grams would be typical for a weekly gain? I'm endlessly relieved she's back on track, but it hurts to know and see her not be as healthy as she should be. I also don't want to feed too heavy too quickly--I've never had a regurge with her and I'm terrified to cause one. So I guess my main three questions are:

1. What is a good average weight for a 3.5 y.o. female (non-breeding) bp?
2. How many grams would a snake gain from a small-rat meal?
3. Would it be better to feed two smaller-sized prey at the same frequency than one larger-sized?

(Btw I switched from feeding in a separate container to feeding in-viv and she strikes much more readily, hasn't refused since. So I know my husbandry was off in that regard. Researching on here has helped in so many ways. Thank you guys so much!)

You aren't going to get a useful average weight that she "should" be at her age because they can vary so much in length. People can tell you what their adult females weigh, but it could easily be a spread of 1000 grams+. Body condition is what you really care about.

For what it's worth - I think feeding weekly is way too often for an adult, even if she's underweight. Possibly *especially* if she's underweight, as counterintuitive as that sounds. The reason is that when they swallow a meal, their internal organs undergo massive changes in order to cope with it. Their stomach and intestines actually get larger and even their heart gets larger in order to cope with pumping blood to those enlarged organs. But it takes a lot of energy to maintain organs capable of digesting an entire meal, if your meals are the size of pythons' meals, and pythons are highly adapted for metabolic efficiency. So after they've digested, all those organs that had to increase in size to cope with the prey shrink back down to their "default" state, which is much smaller. The whole process takes considerably longer than digestion to where you can't see the lump anymore - a week or two I think. If you feed too often, the snake's body never gets back to that between-meals high-efficiency state, or doesn't stay there very long.

If the animal is really at risk of starvation, you maybe need to do everything you can to get some calories into it. But it sounds like your snake isn't actually starving, since she's digested a few meals in quick succession just fine. She's just kind of skinny and has been through a lot lately. Feeding more often is more stress to the system with less time to recover in between.

If you feed less often she'll still put on weight; it might be a bit slower, but it will probably be healthier for her in the long run.

Oh wow I had no idea that the changes lasted longer than a week. So, if the prey is small rats, what would you suggest? A bi-weekly feeding or a 10-12 day feeding interval? I know this is a long-term special care situation so I'll pace her feedings at the rate that's safest for her. She's definitely not emaciated or dehydrated, her shed went perfectly, she's just skinny, as you said.

Recommendations for feeding frequency normal-weight, non-breeding adult ball pythons are kind of all over the map, and unfortunately a lot of the recommendations you see the most either really only apply to growing babies, or come from breeders based on what they have had success with for getting animals up to size and breeding regularly, which is NOT applicable for an adult animal that isn’t breeding and isn’t going to. It may even be more useful to think about the total number of meals per year rather than the actual frequency; ball pythons are very much adapted for long fasts, either to get through times when prey is scarce or because there are times when they have other priorities (i.e, mating, laying, and incubating). For the most part, ten or twelve appropriately-sized meals per year is plenty, and it might not even matter that much whether it’s once a month all year ‘round or every two weeks for six months followed by a six month fast. I’ve seen some recommendations that say every six weeks is plenty. I strongly suspect that a lot of the famous ball python fasts that stress out keepers so much are the result of a feeding schedule that’s appropriate for a growing baby being applied to an adult that just doesn’t need to eat that much.

If it were my snake, I would try every two or three weeks for now and keep an eye on her body condition and weight. As long as she’s gaining ground, she’s being fed plenty even if it’s slow; and once she gets to a weight that looks good for her size, you should reduce her feeding frequency because at that point you don’t want her to pack on more. By all accounts it’s really, really hard and time consuming to get a chubby snake to lose weight because they are so well adapted for conserving energy.

In any case, it’s not a bad thing for your snake to get hungry and start showing hunting behavior for awhile before getting fed again. Keep in mind that for many animals (most animals?) the level of their food drive is calibrated not just to how much food they need, but also to how long it usually takes and how hard they have to work for them to find a meal in the wild. The exercise and activity they get in the process of looking for their food is also an important part of their overall health. The fact that they’re giving you those puppy-dog eyes looking for snacks doesn’t mean they actually need the snack; any vet will tell you they see an awful lot of obese pets and that applies to pretty much every species across the board.

It's been 10 days since I fed her last, and I have a rat thawed rn, so I'll weigh her, feed her, and then go on a 2-week schedule. I only have two snakes so I spend a lot of time observing them--thats actually how I first noticed she was going into shed, she wasn't out cruising/hunting like she usually does when it's around her feeding day. I left her for a few days and when I still didn't see her out I checked on her and she was deep in blue. It's just a slow and steady process to get her in good shape again. I'll probably post in here in a few months to let you give your opinion on how she looks--i don't have any experience with adult bps so I'm not sure what to look for. Thank you for all the info you gave me!

I personally wouldn’t handle her at all on feeding day. BP’s can be shy and any distirbance could cause her not to feed. I’d offer the f/t rat then leave her be for a few days to digest.

Update: she's had 2 more meals, the latest was a medium rat since every store in the area was out of smalls. She took it just fine and as of today is in-blue so will be shedding soon. She's filled out some so her spine looks normal and she's been doing fine on a 12-day cycle.

Great! I would let her go through the shed cycle and then continue with a medium rat every two weeks.