Disclaimer: truly worthy and well-prepared who has faith in these words, I know. No one has to fast. But then there's that fine outward training bit, and I know I could use it, so here are my thoughts if they're worth anything to you. I know I'm not the most authoritative source, but I haven't found that the Fathers have much to offer here. ;)
Pregnant and lactating women are exempt from the fast as the church, like insurance companies, considers them infirm. Makes sense--1.5 meals a day is not enough to grow a baby who depends on you to be his/her sole food source, in or ex utero. On the other hand, there's plenty in my diet that Baby and I could do without. So the basic guide for CSPP fasting that I use is: in penitential seasons, only eat what you should be eating. A lot of recreational consumption goes on around here during normal time, and just eliminating that, sad to say, feels like pretty strict discipline. My preferred form of gluttony is subsisting on that tiny little point of total schwat at the top of the food pyramid to the exclusion of everything underneath it, so Advent and Lent are especially good times for me to work on fixing that (a houseful of kids requiring regular meals helps too).
Other general guidelines by which I inform my CSPP fasting:
--If you have morning sickness, forget the fast. Eat whatever you can stomach (and if you really need to be told this, put up a comment and tell us all what it's like to be a real ascetic).
--No calorie reduction in the first two months postpartum, to ensure that both Mom herself and the dairy are up to speed. If it's a fasting season (I'm going to hit Lent dead-on this year, to my chagrin as this is usually when I catch up on pregnancy weight that's still hanging on) and everyone is feeling good, only eat healthy foods that you should be eating. Much as it pains me to say it, this does not include anything made by my friend Little Debbie.
--If your nursing baby is gaining weight well, and/or is older and eating some other foods, and you're both in good shape otherwise, reducing caloric intake is ok (and even recommended if you could stand to take off a few pounds--and I always find that I'm better at doing this as a spiritual exercise than just because I miss my skinny jeans).
Of course, every mom and every baby and every pregnancy are different. I've put in a few years of this now and feel comfortable with the system I've described, but it wouldn't work for everyone. Nutrition can be a tricky department for baby manufacturers, but we're sinners too and benefit from spiritual exercise and discipline in whatever form is doable. A penitential Advent to all!