I have a rather peculiar question. There are 2 girls my age whom, at around age 19 or 20, suddenly grew a cup size. It was rather spontaneous of an occurance, which makes me curious. Is this merely associated with perhaps some weight gain? Is there some other reason girls who have clearly gone through puberty can have this happen?
Mid and post college years is when a lot of women gain substantial amounts of weight so yes, weight is probably the main answer.
Well then, I must say, both girls gained weight in all the right places 
Gained weight, went on birth control pill, changed to different brand of birth control pill - all these are possible explanations.
Maybe they are responding to massage treatment. 
I didn’t even own a bra until I was 21, and then all of a sudden BLAMMO! boobies gallore.
I think some women just get 'em later - I actually know a few gals like this. (Incidentally, I didn’t really gain much weight around that time, and my body fat % is actually lower now then when I was younger.)
Let’s begin by clearing up a very common misconception: puberty does not work like an on/off switch. When the subject of female sexual development comes up on the SDMB, as it often does, many people seem to use it to mean something like “the onset of menstruation”. That is not what puberty is. It is a complex process that generally takes a decade or more to complete. The most obvious physical changes usually take place during the early and mid teens, but humans do not reach full physical maturity until their early to mid twenties.
So the answer to your question is, your friends had not gone through puberty yet. They were still in the late stages of puberty. Breast development at this age is perhaps not the norm, but it is not especially unusual either.
I’m going to guess birth control pill, or perhaps implants, as those have both been the causes of recent, er, developments in the girls around me. I’ve heard so many girls testify to the former (increasing a cup size when going on the pill), which can come as a delight or a horror, especially when accompanied by backaches and 24/7 nausea and bloating.
Doesn’t pregnancy also sometimes increase breast size? If so then since the pill mimics pregnancy it would seem to make sense that the pill may also do it. Anyone out there that can clear this one up for me.
Um, sometimes boobs just grow later.
No weight gain.
No fat gain.
No implants.
No pill.
No pregnancy.
They just grow later. Period.
When I was about-oh, 17, 18, I went from a B-cup to a C. No weight gain (in fact I was losing weight at the time because I was eating less due to depression), I’ve never been on the pill, or pregnant.
I know a handful of women, myself included, who inflated measureably not long after becoming sexually active. No significant weight gain (we were on a huge campus with no car, so we walked EVERYWHERE), just bigger boobs. At the time, we just put it down to our hormone balances shifting, and didn’t really think much about it.
Oh, by the way, none of us were on the pill at the time, and when I did try the pill, I didn’t notice any changes but lack of cramps.
is 18, a AA, and hopes she will be one of those late developers.
But I have the hips… 