Science of Love, Sex, and Babies

Why is your left breast larger than the right?

Posted in news by jenapincott on April 10, 2009

362537232_hskda-m000x0287x450jpeg2 In BLONDES I mention that, for many women, the left breast is slightly — about 4 percent — larger than the right. The factoid was buried in a discussion about sexual selection, the upshot of which is that breasts are a factor even in cultures that don’t eroticize the chest any more than the face. Breast symmetry is one cue of developmental stability, and large and symmetrical breasts suggest “good genes.” That said, yes, it’s normal to have one breast that is slightly larger than the other. The larger your breasts, the more obvious the asymmetry. For about 65 percent of us, the more voluminous one is the left.

But why?

Breast asymmetry is associated with rapid growth spurts prior to and during puberty. It’s normal for one breast to respond more to surging estrogen levels. Hormonal disruptions and other development problems accentuate the asymmetry. But why the left? No one knows for sure, but there are interesting theories:

* The immune system is known to be functionally asymmetrical, and immune hypersensitivity is higher on the left side of the body. It’s possible that enhanced immunosensitivity has an effect on tissue-plumping growth hormones such as estrogen. The immune system may send to the brain information that is asymmetrically expressed. Immune system asymmetry may also explain why cancers occur 5-10 percent more often in the left breast and inflammation associated with herpes zoster affects the left side more than the right.

* Nursing infants may make the left breast even larger. Although studies have not found a link between a larger left breast and handedness, many women report that they prefer having their dominant right hand free while breastfeeding, which means infants more often nurse on the left breast. Favoring the left breast makes it larger.

Personally, I prefer the explanation my friend gave her curious eight-year-old daughter when she saw that Mom’s left boob was bigger: It’s because I have a really big heart under it.

78 Responses

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  1. mimi said, on April 11, 2009 at 10:02 am

    I, too, like your friend’s explanation! I think she is on to something!

  2. girlslashwoman said, on April 15, 2009 at 2:57 am

    My left boob is almost a cup size bigger than the right but you’ve given me new hope. Maybe when I have kids, I’ll try nursing on the right to even it up a bit ๐Ÿ™‚

    • Melissa said, on September 7, 2011 at 3:32 pm

      hahahaha i was just thinking the same thing haha! i breast feed my son for 11 months but to be honest i switches every 5-10 mins with him! maybe i did favor the left breast more i dont really remember bc it was almost 5 yrs ago BUT next time i have a baby i will most def. favor the right lol ๐Ÿ™‚

  3. […] Why is your left breast often bigger than your right? โ€” beats me โ€” but the lady at the link has a few […]

    • nina said, on November 1, 2010 at 1:03 pm

      my left breast is bigger than my right breast and i experience pain there whenever my period is approaching.i dnt knw if this is normal.

  4. Teri Stoddard said, on April 25, 2009 at 3:47 pm

    Nursing has nothing to do with it. You can see the difference long before any nursing happens.

    • Melissa said, on September 7, 2011 at 3:35 pm

      i was concerned bc i never realized how much the difference between the 2 until 3 days ago.. i always knew one was bigger than the other ut to other ppl it wasn’t noticeable but it really isnow like an entire cup size difference.. idk i think i will ask my doc next visit to be sure lol

  5. Hi, nice post. I have been thinking about this topic,so thanks for writing. I will certainly be coming back to your site.

  6. kat said, on May 5, 2009 at 10:07 pm

    I’m only an A cup but my right is bigger than my left lol.

  7. Wierdo said, on May 8, 2009 at 10:59 pm

    I’m one of the 35% whose right boob is bigger.

    Is that bad?


    • munira's bubble said, on September 30, 2010 at 3:27 pm

      so is mine!

    • steph said, on August 23, 2011 at 3:49 pm

      im the same! my right is noticably bigger.

    • Giraffeluver15 said, on August 29, 2011 at 10:15 pm

      i wouldnt think being the 35% is bad because im that 35% too

  8. cookiemamma said, on May 21, 2009 at 6:10 pm

    MY right boob is bigger too. And that’s funny as they were as close to symmetrical before and I nursed more on the left.

  9. ELISE said, on June 13, 2009 at 10:56 pm


  10. ELISE said, on June 13, 2009 at 10:58 pm


  11. Kate said, on October 21, 2009 at 9:12 pm

    my left is bigger then my right, i felt a bump in the left as well. i am an 18 yr old althlete. i have played sports my whole life. i think that being so active could have caused a tumor or a cycts. i told my mom and she agrees that i shuld get checked. i kno im too young to get breast cancer and it doesnt run in my family but a tumor is just as bad.

    • Jess said, on December 10, 2009 at 5:33 am

      Kate, did you get checked out by your doctor? I have exactly the same situation as you. I do alot of running, left bigger than right, im only 18 too anddd it doesnt run in my family.

      • Mary said, on March 23, 2016 at 12:47 am

        Well I’m screwed. Same situation as you guys except every woman in my blood line has had breast cancer. Somebody please ready my obituary when I die.

  12. mss said, on October 26, 2009 at 1:41 am

    my left boob is bigger than the right…nothing to worry na…..???!!!!

  13. Nikki said, on December 5, 2009 at 2:39 am

    i like your friends explanation as well, have a bit more of a scientific/biologic one to offer though ๐Ÿ˜‰
    as you addressed a bit we all have a dominant and a non-dominant side – one with which we’re stronger (dominant, also called partner side) and one with which we are weaker (non-dominant, also called mother-child side).
    during puberty the growth of breasts can be hampered by difficulties with the respective people (i.e. difficulties with people who are equal to us (partners), which during puberty can be practically any kid at school that has an emotional impact on us, will lead to growth on the dominant side slowing down until the difficulties are sorted out and naturally same with non-dominant side..) is all part of our biology and evolution.

    you will see somewhat of a correlation between right/left handers and left/right breast being larger, but not 100%, the difference is the perspectives of the individual, only taking that into consideration will bring the explanation to a scientific 100%.

    those interested to find out more about these connections and the science behind it check out Meta-Medicine or PNI.. or ask me ๐Ÿ˜‰

  14. Cody said, on December 12, 2009 at 2:12 pm

    That explanation your friend had given her child was cute about her left breast being larger then her right. “it’s because I have a big heart under it.”

    Adorable. =p

    • ammeica said, on September 27, 2011 at 4:30 pm

      hi my name is ammeica chotay ………… my left breast is larger .an the right breast is samll .i need to no how to grow it the same way an i have 3 kids i am 27 yrs old

  15. ---by twilight lover .....brooder said, on January 10, 2010 at 12:36 am

    Same here datz what my aunt telz me. but ma sis says itz cancer…… now who to belive i dnt noe

  16. Donna said, on February 4, 2010 at 5:42 pm

    My left breast is larger, and has been since I started to develope .it bothers me a lot, I am very self concious about it.someone told me about the nursing on the smaller side to even it out, but it dosent work.probably only a cosmetic sergury would be the only thing to fix has caused me a lot of mental stress, and held me back from a lot of things I wanted to do.

    • RoRy said, on July 12, 2011 at 6:16 pm

      i feel the same way

  17. Corpse said, on February 9, 2010 at 2:08 am

    Found an answer to ur problem…

    Your left breast is larger than your right one, because you are
    “right handed” and tend to use your right hand more than the
    left, which means you have more muscle to hold the right
    breast up higher. This can be corrected by “exercising” with
    your left arm more, to balance out.

    Then again, it’s just another unproven idea. I say u ladies should stop worrying about the whole “one is bigger than the other” thing and should simply be thankful that u got a size u can measure. Think of the few unfortunate ones who even after getting pregnant…have breasts smaller than most men.


    • Girlie said, on February 16, 2010 at 6:52 am

      Corpse, this confuses me. Wouldn’t your right breast be larger because you are using that side more, giving it bigger muscles? Huh? I don’t understand how your left would be the one getting larger in that situation.

      • AdviceGirl said, on July 20, 2010 at 1:59 pm

        No ma’am because when you work a muscle on a small scale, it tends to become leaner rather than bigger. Women who do butterflies in the gym can lose their boobs. Boobs have a lot of fat so when the fat burns into muscle from working, it will become smaller and tigher rather than plush like we want our boobs to be. I fully get Corpse’s and totally agree. It makes sense that more people have larger left breasts because more people are right-handed than left-handed. ๐Ÿ™‚

        • amwriter said, on August 9, 2011 at 11:37 am

          I agree with this completely, because I am left handed and have the exact opposite problem everyone else seems to be mentioning., in that my right is nearly a cup size bigger than my left.

  18. sandy said, on February 10, 2010 at 3:56 pm

    hi am 18 yrs old n i have small breasts, but my right breast is smaller than the left one.On top of that am slim so i usually wear tops and jeans but sometimes the difference of my breasts is visible…..plz help me out!!!!!!

  19. Carrie said, on February 20, 2010 at 11:22 pm

    I’ve noticed for a few years that my left is bigger than my right, and i thought for months that it was cancer. I’m so thankful and relieved that it’s not. Knowing that it’s not as uncommon as it sounds is even better…but I noticed a difference when I was only 13. Is that bad that i was so young?

    • n@tali3 said, on March 30, 2010 at 6:36 pm

      carrie im in exactly h
      the same situation, except i am 13 at the moment, my left one seems to be growing more than the right, and it’s not noticable to anyone else, but i am still very un happy about it and i feel self concious. i spend atleast half an hour everyday convincing myself that it’s fine… :/

      • Maddie said, on May 6, 2010 at 10:21 pm

        Yeah, I’m only 13 and it’s very noticeable to me, it seems my left breast is growing a lot more then my right so I looked it up.

  20. jess said, on February 26, 2010 at 2:56 am

    im 14. more recently i have been noticing the difference in my breast’ sizes. one (my left) is a D, but the other (right) is a B.

  21. veronika said, on May 19, 2010 at 6:28 pm

    i thik its really normal you guys shouldnt be gettin worried about that its alright….The reason im sayin this is because im 13 i have a baby and i asked me doctor about one boob being bigger than the other and he said it didnt have notin to do with the nursing that its just part of the gurls development.

    • Mary said, on March 23, 2016 at 12:51 am

      You have a baby at age 13. Nvm this is totally b8. No way this is real.

  22. pbandchutney said, on June 17, 2010 at 4:32 pm

    omg mine are asymmetrical too!! Glad I’m not alone here ๐Ÿ™‚

  23. Brea said, on July 12, 2010 at 5:42 pm

    well im 11. my left boob is bigger than my right, im a blonde and this is scareing me. i noticed it a half a year ago im freakin out! i dont wanna grow up with one boob! it hurts too! if i lie on my belly or bump in to something my left boob is always in pain!

  24. J said, on July 20, 2010 at 1:44 pm

    Has anyone tried putting breast enlargement cream on the smaller boob? I’m thinking of buying some, I am so desperate to try anything. I am also looking into reducing my bigger boob with liposuction. It is supposed to be less traumatic than regular breast reduction and leave less scarring. If you have any other ideas, please share!

    • Roxy said, on October 2, 2010 at 1:00 am

      Don’t try the lipo. I’ve heard that it leaves your breast lumpy and flat like hanging skin. It sags even more after so the difference in breasts is even more noticeable! I wanted to get that done too. They should have surgery to take fat out of one and stick it in the other to even them out but not as harsh as lipo.

  25. AdviceGirl said, on July 20, 2010 at 2:05 pm

    Truth is girls, you all are normal. If you feel something ABNORMAL IN your boob, get it checked out! If you just feel self-conscious because they are different sizes, do not worry. Most girls and women have different-sized boobs. It’s actually a way for men to know they are real and beautiful. And you want your spouse, bf etc. to love you for YOU, not for the job of a surgeon. Don’t worry girls. Just be yourselves. Good men will love you for who you are. ๐Ÿ™‚ Cheer up.

  26. Jackfrog said, on August 19, 2010 at 7:56 pm

    My left ball is bigger than my right ball.

  27. Ellen McAdams said, on August 24, 2010 at 8:43 am

    yay, so noticeable breast

  28. Ting said, on September 1, 2010 at 5:51 am

    My left breast is bigger than my right breast after I pregnant and now my baby is 1 year old.

    Is it I can equalize it if I do more exercise on my left arm?

  29. Janice said, on September 25, 2010 at 10:19 pm

    My left breast was bigger than my right one even before I got pregnant… It got worse now since i’m breast feeding. I try to let the baby feed on my right breast but it does’nt produce enough milk than the left one. I started to get conscious about it when my sis-in-law noticed the difference, i was embarrased by it. I guess the best temperary solution is to buy some kind of silicon to place under the smaller breast just to give that illusion of having a normal size breast… It’s a mind thing you know? Getting worried about it is unneccesary since we can’t really do anything about it unless we undergo surgery.

  30. Stephanie Roberts said, on October 4, 2010 at 8:50 pm

    I found some great products that helped me increased my bust line. Check out my blog for info on how I did it.

    Breast Enlargement Cream,Breast Enlargement Without Surgery, Herbal Breast Enlargement

  31. dpz said, on October 6, 2010 at 1:19 pm

    thanks 4 ur explanation.. my friend is also suffering from the same but one more thing her left breast is bigger and very loose in respect of right.. can u suggest any exercise ???

  32. Contact Paper ยท said, on November 13, 2010 at 11:12 am

    breast reductions would be needed for those women who have accumulated so much fat cells on the breast area `:-

  33. tessa said, on November 26, 2010 at 2:55 am

    Im 13 yrs old i dont feel normal cause my left boob is almost 1 and a hlf a cup sizes bigger and i cry about it alot!!!!!!!!!!!!

    • cali said, on January 24, 2011 at 5:30 pm

      sweetheart.. you are only 13.. you still have so much growing left to do! i would not be too concerned at this current moment.. give yourself time.. to actually grow!!!

  34. saba said, on January 21, 2011 at 3:51 am

    merah aak breast is smail and right breast is big . kia me breast feeding karwasakti ho .

  35. lilly said, on January 25, 2011 at 3:56 pm

    I don’t like the way my breast look as im only 11 and my left boob is 1 cup size bigger than my right i cry about it alot my left is a B cup and my right is a A and i dont know what to do

  36. DR NGOSA P.K said, on February 19, 2011 at 5:40 am


  37. Penny said, on February 22, 2011 at 5:17 am

    For goodness sakes… JUST USE SOME PADS!
    Just understand that you are NORMAL. Stop worrying about being different, because the difference makes up who you are. Be thankful for being born with legs and arms instead of worrying about this stuff.
    Like they said, none of us are perfect.

  38. saanika said, on April 12, 2011 at 4:30 am

    my right breast is smaller than the left due to this rushi is not attracted to me so plz tell me the reason because when i do romance with him he bite me on my breast

  39. Balls of Fury said, on April 27, 2011 at 8:46 pm

    My left ball is bigger than my right one and I ain’t bitchin about it.

    • tatianna said, on June 29, 2011 at 10:58 am

      u don’t have to worry because its hidden inur pants

  40. Cailin said, on May 9, 2011 at 11:49 pm

    Both left and right handed women tend to hold babies on the left side. This is a classic case of post hoc explanation. Left handed women claim that they obviously hold the baby in the strong, dominant hand. Right handed women explain that they want their dominant hand free to do things.

    Some studies have indicated that apes also tend to hold infants on the left – though I believe there is some debate about these results.

  41. sexy said, on May 23, 2011 at 9:38 pm

    i like the big one

  42. sexy said, on May 23, 2011 at 9:51 pm

    me too and im 10 i actully need more growing to do my friend looks at my boobs n say (i can see ur boob its bigger than the other ( i tell her its normal she says no ever girl has same amont of boob she dont kno shes 47 n her boob arnt big at all they havnt grown

  43. Dorothy said, on July 12, 2011 at 8:59 am

    I am 80 years old and have had my left breast bigger than the right one all my life – with no problems. I get a mammogram every year and have never had anything show up. My doctors always said it definitely wasn’t cancer and that it’s not that abnormal. I say if it’s not a problem for you and if there isn’t a HUGE difference, just accept it. There just seems to be a dominant side of the body – feet, ears, eyes, etc., and evidently this is just another example of it.

  44. Rebecca Moran said, on July 29, 2011 at 1:59 pm

    Hi. I’m 18 and find it hard for a bra to fit both sides. I actually looked it up and its something called Poland’s Syndrome.I wear a 40DD bra but my left breast does not fit in it properly. I also find it hard to have a shower as I get neck pains and back pains over it. If any else is as bad as me, let me know.

  45. nIKKI said, on August 7, 2011 at 1:49 pm

    I am eighteen and recently noticed my left breast is still growing but my right is not

    • anna said, on August 19, 2011 at 10:00 am

      same is wid me dont know i nt even go for a spa wid my anty because i feel ashame.. ๐Ÿ˜ฆ

  46. who knew said, on August 15, 2011 at 7:48 pm

    My breasts had always been just about the same size until I hit menopause. Now my left breast is noticeable larger at age 52. Go figure!

  47. my left larger pls help me out

  48. my left breast is larger than the right pls help me out

  49. Horlarhyday said, on August 24, 2011 at 7:37 pm

    My left breast is bigger and larger than the right what do i do

  50. a.j. said, on August 25, 2011 at 10:56 am

    i am 15 yrs old and have gotten hit a lot in the left side of my breast and its a cup size smaller than the one on my right should i be worried since it has grown very little in the past few years

  51. angie said, on September 5, 2011 at 6:23 am

    i am 14 and my right breast is smaller and i feeel terrible with it…i am cryiing alottt…pls help me…plsss
    will the right one grow as the same size as the left??? ๐Ÿ˜ฆ

  52. angie said, on September 5, 2011 at 6:30 am

    my left breast is slightly 1 inch larger

  53. Gorety said, on March 31, 2016 at 11:13 pm

    me 2, so bad_

  54. 0881562480 said, on December 12, 2016 at 4:02 am

    i hope the left breast is bigger than the right one because most men are right handed

  55. said, on January 5, 2017 at 3:04 am Watch and stop at 23:03 exactly. With men’s balls it is the opposite.
    Is it normal to have one testicle that’s bigger than the other?
    Yes. It is common for guys to have slightly different size testicles. Usually, the right testicle is larger than the left. Also, one testicle (usually the left) often hangs lower than the other. Watch from 2:17 onward.

  56. Ben said, on February 13, 2017 at 7:09 am

    it’s just simple. it’s because most men are right handed

  57. keerthana said, on April 25, 2017 at 8:19 am

    Is there any problem to get pragnent

  58. Diana said, on May 30, 2017 at 1:21 am

    I m 17 and have one breast (left ) which is double the size of the other(right) I m very embarrsed about it I dont want to go for any surgerical process please tell me what should I do it is so noticable that it looks really bad I really need help.

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