I do think baby size is pretty genetic, I would guess anyway... my sister, who is normal (on the petite side of normal) and fit, gains 45-50 pounds per pregnancy and has 10lb. + babies. She's married to a big-boned but fit guy whose mother had big babies. I am about the same size as my sister, also fit, but gain about 30 pounds per pregnancy and have 6.5-7.5 pound babies. I would argue we don't do much differently, as we're pretty similar in terms of eating and exercising - both pregnant and non-pregnant, but our husbands are dif't. My husband is considerably leaner built - total runner build, a touch shorter, and his mother had smaller babies.
I was very active during pregnancy number 1, having just ran a marathon two months prior to becoming pregnant. I ran through my pregnancy until 7 1/2 months, and then walked with weights. I was not nearly as active with number 2, but I was very busy with my toddler and school, so I gained a few less pounds, but I actually had a bit of high-ish blood pressure near the end of the pregnancy. I'm hoping to avoid that by remaining more active this time, but I have no ambitions to run any marathons or even half marathons while pregnant! Just long slow jogs sound great to me. I've just run a couple of half marathons in the past 6 months and trained actively for them anyway, so I'm fit going into this pregnancy. I tend to slack off big time though during my first trimester, so I'm hoping to stay active through the tired/sickness/hormonal ups and downs of these first 3 months. I just don't know if I can make any promises, because the temptation to go to bed and eat a McDonald's Super-Value Meal is much stronger than the virtuous choice of a run followed by a nutritious bowl of brown rice and veggies! :)
Good start today though - went for a 4-mile jog this morning to celebrate my positive test!
Hey Stormimay - Congraulations on your recent weightloss... I just want to encourage you to keep working out normally - you're fit now - you should not discontinue the exercise just because you're pregnant. If you're not into running, you could likely safely do almost any kind of aerobic activity for quite some time and then just make room for your belly as it grows.