Q&A: Do your boobs get bigger once you start your period? What about when you're on your period?

For many, breast size will enlarge before they start their period, and is one of many premenstrual syndrome (PMS) symptoms. Hormonal changes that occur before and during your period can cause bloating and swollen breasts among other side effects. Symptoms usually calm down once your period starts.
This is another one of those things that varies from girl to girl. The first time I got my period I didn't notice any changes in my breasts. However, as I've gotten older I can say that sometimes they are way more tender than usual. Breasts grow at a different rate for everyone and I was surprised when mine continued to grow as an adult. I went from a B cup to a C cup in my early 30s and I had already been on my period for 15 years! During the time when you're actually on your period, you may or may not notice a bit of swelling or tenderness. Sometimes you might notice that you have the tenderness without the swelling. If your breasts are especially sensitive during this time, I would suggest trying a more comfortable bra. Perhaps something with more padding can help.
Your boobs won't necessarily get any bigger just because you started your period (bummer!). But since your period is one marker of puberty, it might just coincide with your boobs growing (bonus!). Keep in mind that everybody develops differently, and just because you get your period doesn't mean your boobs are fully developed (and vice versa). Mine didn't reach their current size until many years after I started my period.

My boobs do tend to swell and get a little tender in the days leading up to (and the first few days of) my period. Check out health expert panelist Michelle's answer for more insight into why this happens.