Prenatal Cannabis Exposure Was Linked to Smaller Head Size That Became Significant in Early Adolescence
In a longitudinal study, prenatal cigarette exposure reduced birth weight but children caught up within years, while prenatal marijuana exposure showed no growth effects at birth but was associated with statistically smaller head circumference by early adolescence.
Quick Facts
What This Study Found
Researchers tracked weight, height, and head circumference from birth through early adolescence in children whose prenatal marijuana and cigarette exposure had been documented.
Prenatal cigarette exposure produced clear effects at birth, with lower weight, but these differences disappeared within the first few years as children caught up. By age six, children of heavy smokers were actually heavier than controls. Interestingly, the catch-up appeared related to bottle-feeding or shorter breastfeeding duration among smoking mothers.
Prenatal marijuana exposure showed a different and more subtle pattern. No significant growth differences were visible at birth. However, a trend toward smaller head circumference was observed at all ages, and this reached statistical significance among early adolescents born to heavy marijuana users. Head circumference is used as a proxy for brain size, making this finding potentially concerning.
Key Numbers
Prenatal cigarette effects on birth weight reversed within first few years. By age 6, heavy smokers' children were heavier. Prenatal marijuana: no growth effects at birth, but smaller head circumference reached significance in early adolescence among heavy users.
How They Did This
Longitudinal study tracking children from a low-risk, predominantly middle-class sample from birth through early adolescence. Prenatal marijuana and cigarette exposure was ascertained prospectively. Growth measures (weight, height, head circumference) were tracked at multiple time points.
Why This Research Matters
The delayed emergence of the head circumference finding is important: prenatal marijuana effects may not be visible at birth but could manifest as development progresses. This pattern of "sleeper effects" became a recurring theme in prenatal cannabis exposure research.
The Bigger Picture
The concept of delayed or "sleeper" effects from prenatal cannabis exposure has been supported by subsequent research showing cognitive and behavioral differences that emerge in later childhood and adolescence. The head circumference finding added biological plausibility to these later-emerging effects.
What This Study Doesn't Tell Us
Head circumference is an imprecise proxy for brain development. The effect reached significance only among heavy users in the early adolescent subgroup. Self-reported prenatal substance use may be inaccurate. The predominantly middle-class sample may not represent higher-risk populations.
Questions This Raises
- ?Does the smaller head circumference translate to measurable cognitive differences?
- ?Does the effect continue into later adolescence and adulthood?
- ?What is the biological mechanism by which prenatal cannabis affects head growth years after exposure?
Trust & Context
- Key Stat:
- Prenatal marijuana was associated with statistically smaller head circumference by early adolescence
- Evidence Grade:
- A longitudinal cohort study with prospective exposure assessment. Good design for tracking developmental trajectories, but the marijuana finding was limited to heavy users and one growth measure.
- Study Age:
- Published in 1999. This cohort has continued to be followed, providing additional data on prenatal cannabis exposure effects through adolescence.
- Original Title:
- Growth from birth to early adolescence in offspring prenatally exposed to cigarettes and marijuana.
- Published In:
- Neurotoxicology and teratology, 21(5), 513-25 (1999)
- Authors:
- Fried, P A(2), Watkinson, B(2), Gray, R
- Database ID:
- RTHC-00078
Evidence Hierarchy
Follows a group of people over time to track how outcomes develop.
What do these levels mean? →Frequently Asked Questions
Does smoking marijuana during pregnancy affect the baby's growth?
In this study, no growth effects were visible at birth, but children of heavy marijuana users had statistically smaller head circumference by early adolescence, a possible indicator of brain development effects.
How did marijuana compare to cigarettes?
Cigarette effects were immediate (lower birth weight) but children caught up. Marijuana effects were delayed, with no birth differences but a head circumference signal emerging years later.
Read More on RethinkTHC
Cite This Study
https://rethinkthc.com/research/RTHC-00078APA
Fried, P A; Watkinson, B; Gray, R. (1999). Growth from birth to early adolescence in offspring prenatally exposed to cigarettes and marijuana.. Neurotoxicology and teratology, 21(5), 513-25.
MLA
Fried, P A, et al. "Growth from birth to early adolescence in offspring prenatally exposed to cigarettes and marijuana.." Neurotoxicology and teratology, 1999.
RethinkTHC
RethinkTHC Research Database. "Growth from birth to early adolescence in offspring prenatal..." RTHC-00078. Retrieved from https://rethinkthc.com/research/fried-1999-growth-from-birth-to
Access the Original Study
Study data sourced from PubMed, a service of the U.S. National Library of Medicine, National Institutes of Health.
This study breakdown was produced by the RethinkTHC research team. We analyze and report published research findings without making health recommendations. All interpretations are based solely on the published abstract and study data.