Yep. You’re not imagining it. Your boobs are bigger when you have a boy. And evidence backs that up.

Sometimes changes in your boobs can be an early sign of pregnancy. It seems like even looking at your boobs can make them hurt, and they often get firmer than you’re used to.

Hormones start working fast when you get pregnant—especially on your breasts in preparation for breastfeeding. Fatty tissue grows and blood flow increases to help the milk ducts and mammary glands grow. According to the March of Dimes, by the time you are about six weeks pregnant, your breasts can grow a full cup size or more, and on average, by your ninth month, you gain two pounds just in your boobs.

Changes in your boobs can tell you more than just if you are pregnant—they can tell you if you are having a boy or a girl.

Studies show changes in breast size can predict baby’s sex

Wondering if your breast size increase during pregnancy means you’re having a boy or girl? Recent research may have the answer. In a small study published in the American Journal of Biology, 93 women were recruited to participate in a three-stage longitudinal study. They received 3D breast scans to calculate their breast volume at the 12th, 22nd and 32nd week of their pregnancy. The values were compared between women who were having a boy and those who carried a girl.

Results indicated that a greater increase in breast size—not just size alone—predicted that a mother was more likely to be having a boy.

This same study revealed that if your boobs were asymmetrical before your pregnancy (like 88% of women), they are likely to even out with pregnancy, but that symmetry has no bearing on whether you are pregnant with a boy or a girl.

Baby boys may require different nutritional needs

In a study of 8,556 babies, it was discovered that at birth, boys were on average 7 ounces heavier than girls, possibly requiring different nutritional needs for breastfeeding.

Breast milk quality can affect your baby’s health, growth and development, so researchers set out to quantify nutrient and energy content. In a small study, breast milk from 25 healthy, well‐nourished women with babies aged 2- to 5-months was analyzed. Potential sources of variation in breast milk quality, like feeding patterns, infant sex and the mother’s breast growth during pregnancy, were considered. After controlling for time since the last feeding (since breast milk composition changes throughout the day) researchers found that mothers of boys produced milk that had 25% greater energy content than mothers of girls.

Related: Newborn breastfeeding: Your guide to the first week of nursing

Another analysis of 500 samples of breast milk from 61 mothers over 12 months echoed these findings by revealing that breast milk has more fat when you have a boy. Seventy-seven mothers at 4- to 8-weeks postpartum provided breast milk samples that were analyzed for their composition. It was determined that breast milk from mothers of boys had a higher carbohydrate content, too.

Researchers also found that participants’ pre-pregnancy and current nutritional status affected the composition of their breast milk. This suggests that what is in the breast milk you make could be determined during your pregnancy. The study authors wrote that breast milk “may have [a] unique compositional profile for every mother-infant dyad.”

So even before they are born, and no matter if they are a boy or a girl, your baby already may be telling you exactly what they need.

4 products to support you on your breastfeeding journey

Earth Mama organic nipple butter

Earth Mama


Organic Nipple Butter

Earth Mama’s Organic Nipple Butter is a fan favorite for a reason. It’s free of petroleum, parabens and lanolin, and applies like a dream, working quickly to soothe your sore and raw nipples. You can check out our midwife’s glowing review here !

nunona mama balls



Nourishing Mama Balls

These energy balls were created to support your nutritional needs as you make super-nutritious breast milk. The box contains 28 bites that make nourishing your body a breeze—and are totally delicious, too.

willow gen three breast pump 1



Wearable Breast Pump

Is the Willow pump worth it? In a word, yes. (Check out our review here !) The wearable electric pump allows you to pump untethered and in comfort—it might even help you pump more milk, as users saw a 20% increase in volume!

Ritual prenatal vitamin



Essential Prenatal

Ritual’s prenatal is built on an “everything you need, nothing you don’t” philosophy. Each of the delayed-release capsules are easy to absorb and contain just 12 essential nutrients including methylated B vitamins, bioavailable iron and 350 mg of DHA, an omega-3 fatty acid that’s necessary for fetal brain and eye development. And since the best vitamins are the ones you’ll actually want to take, these come with a clever citrus tab inside the bottle to make the whole experience even better.

Read more about it here.


Bzikowska-Jura A, Sobieraj P, Szostak-Węgierek D, Wesołowska A. Impact of Infant and Maternal Factors on Energy and Macronutrient Composition of Human MilkNutrients. 2020;12(9):2591. Published 2020 Aug 26. doi:10.3390/nu12092591

Fischer Fumeaux CJ, Garcia-Rodenas CL, De Castro CA, et al. Longitudinal Analysis of Macronutrient Composition in Preterm and Term Human Milk: A Prospective Cohort StudyNutrients. 2019;11(7):1525. Published 2019 Jul 4. doi:10.3390/nu11071525

Galbarczyk A. Unexpected changes in maternal breast size during pregnancy in relation to infant sex: an evolutionary interpretationAm J Hum Biol. 2011;23(4):560-562. doi:10.1002/ajhb.21177

Powe CE, Knott CD, Conklin-Brittain N. Infant sex predicts breast milk energy contentAm J Hum Biol. 2010;22(1):50-54. doi:10.1002/ajhb.20941

Rohrich RJ, Hartley W, Brown S. Incidence of breast and chest wall asymmetry in breast augmentation: a retrospective analysis of 100 patientsPlast Reconstr Surg. 2006;118(7 Suppl):7S-17S.

Żelaźniewicz A, Pawłowski B. Breast size and asymmetry during pregnancy in dependence of a fetus’s sexAm J Hum Biol. 2015;27(5):690-696. doi:10.1002/ajhb.22716

A version of this story was originally published on March 3, 2021. It has been updated.