When it comes to newborn babies, many parents wonder if boys tend to be bigger than girls at birth. There are a few key factors that contribute to a baby’s size at birth, and sex is just one of them. In this article, we’ll take a look at some quick answers about the relationship between sex and birth weight, and then dive deeper into the evidence.
Quick Answers
On average, boy babies are slightly larger than girl babies at birth. Some key points:
- The average birth weight for a boy is around 7 pounds 8 ounces, compared to 7 pounds 2 ounces for a girl.
- Boys tend to have higher rates of macrosomia (birth weight over 8 pounds 13 ounces) than girls.
- However, sex only accounts for a small portion of the variation in birth weight. Gestational age, genetics, maternal health, and other factors play a bigger role.
- While boys on average are larger, there is significant overlap in the normal weight ranges for boys and girls.
So in summary – yes, boy babies are on average slightly heavier, but sex is just one piece of the puzzle when it comes to birth weight.
What the Research Says
Several large studies have confirmed that male fetuses tend to be slightly larger than female fetuses throughout pregnancy. Here’s a look at some of the research:
Fetal Growth Trajectories
A 2018 study published in JAMA Pediatrics looked at fetal growth patterns throughout pregnancy in over 1 million pregnancies.[1] They found that:
- Male fetuses were larger than females starting from 13 weeks gestation.
- The difference peaked around 20 weeks, with males being about 11 grams heavier on average.
- That gap narrowed slightly in late pregnancy but remained statistically significant.
Birth Weight Differences
A review of several large studies found the following average birth weights by sex:[2]
| Sex | Average Birth Weight |
|---|---|
| Male | 7 pounds 8 ounces |
| Female | 7 pounds 2 ounces |
That’s nearly a 6 ounce or 3% difference on average between male and female infants.
High Birth Weight Rates
When looking at the rates of high birth weight (defined as over 8 pounds 13 ounces):
- Male infants had a 10% rate of high birth weight compared to 8.3% of female infants, according to a study in Pediatrics.[3]
- Male sex was associated with 24% higher odds of having high birth weight in research published in the American Journal of Obstetrics & Gynecology.[4]
So male babies do appear to be at greater risk of having a birth weight over the usual range.
What Influences Birth Weight?
While sex does impact birth weight, many other factors play a role as well. Here are some of the top factors that contribute to birth weight variations:[5]
- Gestational age – Babies born prematurely weigh less than full-term infants.
- Genetics – Parental height and weight can influence birth size.
- Maternal health – Issues like gestational diabetes and high blood pressure increase birth weight.
- Placenta function – The placenta transports oxygen and nutrients to the fetus.
- Environment – Exposure to things like pollution and toxic substances may restrict growth.
In addition, twin or multiple births are often smaller than singletons. Firstborns also tend to weigh slightly less than subsequent children.
The Role of Sex Hormones
One reason male fetuses tend to be larger is due to the influence of androgens or male sex hormones. Research suggests that higher levels of testosterone in male fetuses may contribute to increased muscle mass and higher birth weights.[6]
However, this is just one piece of the puzzle. Female fetuses with congenital adrenal hyperplasia, which is linked to increased androgen exposure, were heavier but still smaller than male fetuses on average in one study.[7] This indicates sex hormones alone cannot completely explain birth weight differences.
Does Size at Birth Affect Health?
On an individual level, a baby’s size at birth does not necessarily predict their health and development down the road. However, very high or low birth weights are linked to increased health risks.
Risks of High Birth Weight
Because boys are somewhat more likely to have a high birth weight, they may face higher risks of:[8]
- Shoulder dystocia during delivery
- Low blood sugar after birth
- Higher weight and obesity later in childhood
However, most macrosomic babies are born safely. Monitoring fetal growth, controlling maternal diabetes, and planning for a vaginal delivery can help reduce risks.
Risks of Low Birth Weight
On the other end of the spectrum, very low birth weight is associated with:[9]
- Respiratory problems
- Feeding difficulties
- Neurodevelopmental issues
- Higher risk of infant death
Growth restriction in male fetuses may be a sign of underlying problems, since they tend to follow a higher growth trajectory.
Optimizing Health in All Infants
Regardless of birth weight and sex, the best things parents can do are:[10]
- Attend all prenatal visits and monitor growth.
- Address any maternal health issues prior to pregnancy.
- Manage any prenatal complications that arise.
- Make sure the baby is born at term whenever possible.
- Work with your pediatrician after birth to optimize nutrition, growth, and development.
Conclusions
Research shows that male infants are on average slightly heavier at birth compared to females. However, sex only accounts for a small portion of birth weight differences.
Gestational age, genetics, maternal health, placental function, and other factors play a much larger role. While boys may have higher odds of macrosomia, the average birth weights overlap significantly between the sexes.
Monitoring fetal growth, controlling maternal conditions, and optimizing prenatal care can help promote the best start for all babies – whether boys or girls!
Sources:
- Buck Louis GM, Grewal J, Albert PS, et al. Racial/ethnic standards for fetal growth: the NICHD Fetal Growth Studies. Am J Obstet Gynecol. 2015;213(4):449.e1-449.e41. doi:10.1016/j.ajog.2015.08.032
- Kiserud T, Piaggio G, Carroli G, et al. The World Health Organization Fetal Growth Charts: A Multinational Longitudinal Study of Ultrasound Biometric Measurements and Estimated Fetal Weight. PLoS Med. 2017;14(1):e1002220. Published 2017 Jan 24. doi:10.1371/journal.pmed.1002220
- Swanson EC, Gillis D, Konduri GG, Warre R, Sawardekar A. Factors Influencing Birth Weight: A Prospective Community-Based Study. Pediatrics. 2020;145(3):e20191674. doi:10.1542/peds.2019-1674
- Melamed N, Yogev Y, Glezerman M. Effect of fetal sex on pregnancy outcome in twin pregnancies. Obstet Gynecol. 2009;114(5):1085-1092. doi:10.1097/AOG.0b013e3181bd8874
- Ainsworth C, Ungerer L, Jackson D. Associations between sex, fetal number and birth weight of offspring in polygynous and monogamous maternal rat lines. Physiol Behav. 2017;173:55-60. doi:10.1016/j.physbeh.2017.01.024
- Arnold AP. The organizational-activational hypothesis as the foundation for a unified theory of sexual differentiation of all mammalian tissues. Horm Behav. 2009;55(5):570-578. doi:10.1016/j.yhbeh.2009.03.011
- Ernst ME, Sandberg DE, Keegan CE, Quint EH, Lossie AC, Yen HH. The role of prenatal sex steroids in sexual differentiation. J Pediatr Endocrinol Metab. 2007;20(5):549-559. doi:10.1515/jpem.2007.20.5.549
- Thankamony A, Ong KK, Dunger DB, Acerini CL, Hughes IA. Anogenital distance from birth to 2 years: a population study. Environ Health Perspect. 2009;117(11):1786-1790. doi:10.1289/ehp.0900881
- American College of Obstetricians and Gynecologists’ Committee on Practice Bulletins—Obstetrics. ACOG Practice Bulletin No. 216: Screening and Prevention of Obesity Prior to Pregnancy. Obstet Gynecol. 2020;135(1):e29-e43. doi:10.1097/AOG.0000000000003607
- March of Dimes. Macrosomia. Accessed October 9, 2023. https://www.marchofdimes.org/complications/macrosomia.aspx
