I'm not sure if breastbinding would be an option for MTF's who are not out yet or can't fully transition, either with breast implants or through hormones.

This woman shows the difference in the look of breasts before and after breast binding:

Option, yes, but I believe binding breasts that are trying to grow is counterproductive. Also, binding improperly can hurt you, so read up on that.

Hugs, Devlyn

It's not a good idea; it can permanently damage breast tissue, *especially* developing breasts. Obviously that's not a concern for people who aren't worried about having perky full breasts later on, but for a trans woman (who likely does want them to be in good shape eventually)...

My suggestion is a sports bra under a T-shirt with a loose shirt on top of that. I was able to hide up to 42C that way.

Putting the cart before the horse me thinks...

I have a B cup and even in a bra you really really have to look hard and at different times to even begin to know for sure. Sure, I know when I see like how prominent they are thanks to the seat belt. But I have inside info. I always wore loose to baggy clothes and still do. Only in a tight tee shirt can someone else strongly suspect

If I had a full B life would be a bit different. THen I'd have to always rely on my Plan B, which are my cami shapers. They do some flattening without really binding and various cup styles are available. If I had a really really full B to full C then I'd think that even with my bald head I reached the point of Male Fail and likely only fooling myself

I  think  the reality  of  the  question,  is,  if  your  taking  hormones,  and  have  significant  breast  growth,    shouldn't  you  consider  simply  being  "out",  and getting  on  with  life  out  of  the closet? 
  there  are  many  milestones,  but  that  question  suggests  you  have  reached,  or  will  shortly,  a  huge  one. 
  changing  the  body  should  have  come  with  a  plan,  and  a  commitment,  that  at  some  point  you  will  become  you.  that  means  realizing  things  are  progressing,  planning  and  exicuting  that  plan  to  get  out  of  the  shadows,  and  enjoying  that  freedom. 
 

I wouldn't worry about it. I wear a sports bra and people still see me as a guy, and I've had numerous people (who found out about my past) say "but...you don't have boobs"even though I very much do.

I would recommend against it. You may end up squishing your breasts and I'm sure you don't want that. As the others have mentioned above a sports bra under a baggy shirt usually does the trick - as will wearing jackets.