Most heat-exposed workers had urogenital symptoms, which might be associated with pregnancy-related complications
Photo: iStock The increasing frequency of heatwaves in tropical countries like India can impact the health of pregnant women. Occupational heat exposure poses, in particular, a threat to the health of working mothers and the foetuses in their wombs, doubling the risk of miscarriages or stillbirths.
A study based in six districts of Tamil Nadu followed pregnant women performing heavy manual work in the informal sector, primarily in agriculture, brick kilns, salt pans and construction. They worked in the scorching heat from 6 am to 5 pm, lacking access to toilets and shade, carrying water bottles because access to safe drinking water at the site is mostly unavailable.
Researchers at the Sri Ramachandra Institute of Higher Education and Research in Chennai recruited 800 women eight to 14 weeks into their pregnancy, half of whom worked in the formal sector and were included for comparison. Their jobs allowed scheduled breaks for rest, and access to water and shade.
The heat-exposed cohort showed a higher probability of adverse health outcomes related to pregnancies, the study established. Among them, miscarriages occurred in 5 per cent, stillbirth or preterm birth occurred in 6.1 per cent and low birth weight occurred in 8.4 per cent of pregnancies.
For women not exposed to heat as part of their job, the prevalence of miscarriage was 2 per cent, stillbirth or preterm birth was 2.6 per cent and low birth weight was 4.5 per cent, the researchers found.
The Wet Bulb Globe Temperature (WBGT) was used to quantify heat exposure and consequent health outcomes. The yearly average WBGT was 27.1°C for the study period between 2017-19 and 2021-22.
Besides temperature, WBGT takes into account wind speed, cloud cover, sun angle and humidity. The Threshold Limit Value (TLV) is the value at which the workers can be repeatedly exposed to WBGT, provided they are acclimatised to the weather. Heat exposure exceeding TLV in early pregnancy increases the risk of miscarriage.
The researchers used core body temperature as a physiological heat strain indicator. A rise in core body temperature was recorded in 17.4 per cent of the exposed workers and 29.6 per cent of workers experienced moderate dehydration, they found.
High heat is the main cause of limiting the foetus’s growth in the second and third trimester, according to the study. As organs are formed in the first trimester, heat exposure puts their offspring at risk of congenital birth defects.
WBGT ranged from 22-29.7°C during the first trimester, 22-32°C during the second trimester and 22-29.8°C during the third trimester, they calculated.
Source: Sri Ramachandra Institute of Higher Education and Research in Chennai
The researchers took 15-20 measurements every day on an hourly basis, which were then averaged to attribute the exposure of workers. This was to demonstrate that workers had high heat exposure throughout pregnancy.
Pregnant women in their first trimester exposed to heat above TLV had a 3.1 times higher risk of pregnancy-related complications. In the third trimester, women with heat exposure above WBGT TLV had twice the risk of adverse birth outcomes.
However, for all three trimesters, heat exposure was associated with a 2.2 times increased risk of adverse foetal or maternal outcomes.
Despite some investigation, scientists and physicians still lack a complete understanding of the physiological mechanisms behind the impact of heat exposure and its effects on pregnant women. Previous research has indicated that increased temperatures prevent the foetus from meeting its growing physiological requirements.
While maternal heat stress transforms into foetal heat stress, there could be other biological mechanisms at play that need more exploration, a 2022 study based in Gambia by the London School of Hygiene and Tropical Medicine noted.
This study looked at pregnant subsistence farmers in the West Kiang region and concluded that heat stress was significantly associated with foetal strain. However, a lot remains unknown.
“We suggest that an important physiological factor to consider in future work is the role of the diversion of blood from the placenta to the skin, which occurs at lower core temperatures than those that have been previously highlighted as teratogenic. This factor has been well recognised in maternal exercise and signposted as a potential issue in maternal thermoregulation, but hasn’t been thoroughly investigated,” the paper said. Teratogens are drugs, chemicals, infections and metabolic agents that can impair foetal development. Heat can act as a teratogen.
The Chennai study reported that a majority of the heat-exposed workers had urogenital symptoms, which might be associated with pregnancy-related complications.
Heat protection for pregnant workers is a critical issue, adaption measures need to be prioritised amid a 2.5°C global temperature rise in the offing, the researchers argued.
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