You are still eating for two, so do not eat at a caloric deficit. Eat to satisfy your hunger.
And most especially, do not fast. Women have died from trying to fast while pregnant or lactating and eating a ketogenic diet. There is a phenomenon called “euglycaemic ketoacidosis” that can occur, and it is potentially fatal. Pregnant or lactating women should not try to fast, for that reason.
The fat loss promoted by a ketogenic diet is not from cutting calories, but from cutting carbohydrate. When we eat a significant amount of carbohydrate, our body produces insulin to remove the glucose (carbohydrate molecules are nothing more than long strings of glucose molecules bound together) from the bloodstream. Some of it is burned in the muscles, the rest is stored as fat. And as long as insulin remains high, that fat is trapped in our fat tissue, and cannot be metabolised. The solution is to eat a very low amount of carbohydrate (or even none at all), so as to keep insulin as low as possible for as much of the day as possible.
As long as you are eating a minimal amount of carbohydrate, you can trust your appetite to set your caloric intake to the proper level, so that you can eat to satisfy your hunger, and still metabolise any excess fat that your body has stored. On the other hand, if you skimp on calories, your body will lower its metabolic rate to compensate, and will hold on to its fat store in order to see you through the famine. This makes weight loss difficult in the best of circumstances, and is an especially bad idea while pregnant or lactating. Right now you need protein and fat, to feed both yourself and the baby, so trust your body and eat to satisfy your hunger.