Scientists say whilst air pollution can reach the placenta, further study is needed to determine whether this affects the foetus.
Particle transfer across the placenta from factory smokestacks and car exhaust fumes have previously been linked to premature births and low birth weight, but scientists are still trying to understand why.
An observational study published in Nature Communications looked at 28 women who had donated their placentas.
Professor Tim Nawrot, of Hasselt University in Belgium, and colleagues used high-resolution imaging to detect black carbon particles in placentae collected from five pre-term and 23 full-term births.
The study involved 10 mothers who had been exposed to high levels of residential black carbon particles (2.42 micrograms per cubic metre), against 10 mothers exposed to lower levels (0.63 micrograms per cubic metre).
Particles of black carbon had been found on the baby’s side of the placenta, but researchers could not confirm whether this had affected the baby.
Researchers say it is important to understand how these particles affect pregnancy – through direct effects on the foetus or indirect effects through the mother – to improve pregnancy care in polluted areas.
The authors wrote: “Our results demonstrate that the human placental barrier is not impenetrable for particles.
“Our observation based on exposure conditions in real-life is in agreement with previously reported ex vivo and in vivo studies studying the placental transfer of various nanoparticles.”
The placenta is intrinsic for nourishing a developing foetus, and Andrew Shennan, Professor of Obstetrics, King’s College London (KCL), called these findings “a concern”.
“Small particles, such as through smoking, can cause considerable disease related to the placenta,” he said.
“Their possible effects on the baby and mother warrant further investigation.
“The placenta is the interface between mother and baby and is key to nourishing and supporting all the needs of the baby.
“Both the function and structure of the placenta is important, not only to the baby’s growth and wellbeing, but also to that of the mother.
“High blood pressure and fits in pregnancy have been linked to household pollution.”
A World Health Organisation study shows 90% of the world live above acceptable air pollution levels.
Across England, Wales and Northern Ireland alone, around 2,000 locations have recorded unsafe levels of air pollution.
It’s been described by the UK government as the biggest environmental threat in the country, with between 28,000 to 36,000 deaths a year due to long-term exposure to air pollution.
The government is investing £3.5 billion to tackle poor air quality through transport with aims to halve harm to human health from air pollution in the UK by 2030.