My mom gained less than 30 pounds each time she was pregnant, and we were over eight and over ten pounds at birth. Yet some women gain twice or more that weight and end up with babies who are smaller. Clearly how much weight the woman gains isn’t the only thing that influences the size of the newborn at birth. What are the other factors? In full-term newborns, I mean.
Same things that affect adult size, mostly a combination of nutrition and genetics. I don’t know if there’s any correlation whatsoever between mother’s weight gain and baby weight. Also, gestational diabetes can significantly increase birth weight; IIRC most world-record birth weights were associated with GD. Also IIRC the mother smoking or taking drugs can lower birth weight.
IANAD, but in my observation there seem to be so many factors it wouldn’t pay to count them! General factors of inheritance, plus nutrition and other environmental factors are primary. Anecdote != data, but I’m 5’ 7" and both my babies were smaller than either of my 5’ 2" sister’s. We were both similarly careful about diet and nutrition. She smoked, I didn’t.
When I had a hysterectomy several years ago the doctor commented that I had a smaller uterus than average; perhaps that was a factor, maybe not.
A wide range of factors will influence how long a baby is carried, and the last few weeks are spent on finishing touches and weight gain, so a child that stays in utero for a few more days will, all other things being equal, will likely be larger.
Also based only on my observation, birth size doesn’t necessarily relate at all to the person’s adult height or weight. I’ve known people who were born small but grew fast and vice versa. I’ve also know women who gained relatively small amounts of weight but had reasonably large kids.
And yet smoking is supposed to be linked to lower birth weight. What build are your respective partners?
Pretty much everyone I know had a baby last year. Some surprising results. The smallest baby was born to one of the largest couples - 5lb4oz, if I recall correctly, but he was 4 weeks early. Still, not huge even taking that into account. Another friend had a 7lb3oz baby 3 weeks early. She’s a diabetic and was on insulin through her pregnancy so she was never going to have a small one, but that wasn’t too far off a normal weight. The largest baby was something like 13lb, but it turns out his mother had undiagnosed gestational diabetes. She was tested for it at week 28, but doctors think (by the baby’s size) that it must have set in around week 32. My baby was a week late and 7lb4oz after they told me she’d probably be at least 10lb based on the 38 week ultrasound. My nephew was a surprising 9lb10oz, but he’s quite long and looks like he’ll take after his tall uncles on the other side of the family. Had we taken bets on sizes and birth weights, I’d be out a bit of cash now.
My brother-in-law is taller and generally larger than my husband, so that could be a factor. None of the children involved grew to be as tall as either my brother-in-law or me.
My daughter was born 4 weeks early, she was 4lbs 12 oz. The last months are when babies seem to put on the pounds, I was having an extremely hard time eating the last couple months I was pregnant, my doctor told me she was about 3lbs a month before she was born and that if I did not put on 2lbs a week, he would take her out of me. He didn’t care how I did it. He suggested milkshakes, ice cream, ANYTHING. Turns out, fudge rounds have more calories in a smaller package. 
I ate a lot of them and drank ensures like they were water in a desert. I put on 1 1/2 lbs the first week. For the record, I was considered underweight when I got pregnant and put on exactly the 35lbs they recommended and still had a month to go.
Those last 3 months make a huge difference, I ate like a pig the first 4 months.
FTR: I’m 5’7" and my husband is 6’3".
@Auntbeast:
The way you say it, getting pregnant sounds like heaven. Where do I sign up?
That is something I know about. Over 8lbs and 22" at birth, and I’m not even 5’4" now. By the time I was three I was deemed “small for my age” by my pediatrician. Too bad I can’t remember the only time in my life I was tall!
Yep, my son was 9 1/2 pounds at birth (actual delivery nurse quote at birth: “OMG, what a huge baby!”), and grew like a monster through age two, and was pretty much the largest, heaviest kid I saw in his age group. At 8 months or so, picking him up was like hefting a couple of cinder blocks.
Now he’s ten, and he’s about smack in the middle of the height/weight charts, and well on his way to getting my meager 5’ 7" stature, rather than his mom’s height genes (she’s she’s 5’ 9", but has brothers well over 6’). He’s currently one of the shortest (though not skinniest, he’s pretty darn wiry) among his friends.
It was!
I had a very easy pregnancy, considering I was high risk due to my age and every time I went to my doctor, he’d scare the crap out of me. I had about 4 bouts of morning sickness that were mild at best.
I was only in labor for 20 min. I got the epidural. My child was tiny. I tend to hesitate in relating my experience because most child bearing women want to kill me. Did I mention I was back down to fighting weight about 3 months later? When I talked to my girlfriend, she said I had no idea what giving birth was like, I gave birth to an orange. No stretch marks, boobs are still great. I’m tellin’ ya, I believe most women lie. If everyone had as easy time being pregnant as I did, everyone would do it.
Although I have to tell you, I haven’t had a fudge round since. I was such a wacko, being underweight most of my life, I really had no idea what had a lot of calories. I was reading the label on EVERYTHING.
Oh yeah, and breastfeeding was a piece of cake. I was kinda pissed when we had to switch her to bottles, bottles are such a pain in the ass.