Mean birth weight at term significantly increased globally since 1950

Key takeaways:

• Since 1950, mean birth weight at term significantly increased worldwide each year.
• Results remained consistent in several review models.

Worldwide, mean birth weight at term has increased by more than 7 g each year — a total of about 1 lb — since 1950, according to a systematic literature review published in the American Journal of Obstetrics and Gynecology.

“From mid-20th century to our days, various studies from different regions, including the United States, Canada, Chile, India, China, Israel, Denmark, the United Kingdom, Australia and many others, have explored trends in birth weight,” Giulia Bonanni, MD, postdoctoral research fellow in the division of fetal medicine and surgery at the Maternal Fetal Care Center at Boston Children’s Hospital at Harvard Medical School and the department of women’s and child health sciences and public health at IRCCS A. Gemelli University Polyclinic Foundation at the Catholic University of Sacred Heart, Rome, and colleagues wrote. “These studies often report an increase in mean birth weight over time. Yet, a comprehensive synthesis of global literature has been lacking.”

Bonanni and colleagues systematically reviewed 29 ecological and prospective or retrospective observational studies published from 1950 to September 2023 from PubMed, Embase and Web of Science. All studies reported data on mean birth weight at term per year for more than 183 million infants in different countries worldwide. Researchers evaluated global trends in mean birth weights at term as reported in the studies.

Overall, 48.3% of studies were hospital-based, 44.8% used national data and 6.9% used municipality, community or regional data. In addition, 31% of studies were conducted in North America, 27.6% in Asia, 27.6% in Europe, 6.9% in South America and 6.9% in Oceania.

Over time, in the univariate linear regression model, mean birth weight at term increased by 4.74 g per year (95% CI, 3.95-5.53). In a second model using binary dummy variables representing different continents to address geographical variations, researchers confirmed this trend and observed a mean birth weight increase of 3.85 g per year (95% CI, 2.96-4.74).

When focusing on records from 1950 onward, researchers observed a robust increase of 7.26 g per year (95% CI, 6.19-8.33) in mean birth weight at term. Researchers also observed an increase of 1.46 g per year (95% CI, 0.74-2.18) in mean birth weight at term when adjusting for the number of infants in each study.

Researchers observed no statistically significant relationship between time and mean birth weight at term in a subgroup analysis focusing only on data from national sources.

“Future research should focus on conducting large-scale prospective cohort studies with attention to gestational age distinctions. Additionally, comparative studies examining the impact of different health care systems, socioeconomic factors and geographical factors on birth weight trends would contribute valuable insights,” the researchers wrote. “These endeavors will not only address the limitations highlighted in our study but also provide a more nuanced perspective on the implications for maternal and child health.”