Biological Impacts of Rising Temperatures on Maternal, Fetal, and Newborn Health: A Cohort Study (BIRTH Cohort)

Background

Anthropogenic climate change has led to a significant rise in global temperatures, resulting in more frequent, intense, and prolonged heat events. This has profound health implications for vulnerable populations, particularly in low-resource settings. Pregnant women are particularly susceptible, as heat exposure can disrupt maternal thermoregulation and fetal development, potentially leading to complications such as preeclampsia, gestational diabetes, hypertensive disorders, cardiovascular events, and increased risks of preterm birth, low birth weight (LBW), small-for-gestational-age (SGA) births, and stillbirths.

Studies have indicated a 1.04-fold increase in preterm birth odds per 1°C rise in temperature, with even greater risks during heatwaves. Additionally, associations have been observed between heat exposure and stillbirth, congenital anomalies, and gestational diabetes. Despite growing epidemiological evidence, there is limited data on the biological mechanisms driving these associations, particularly in low- and middle-income countries (LMICs).

Several biological pathways have been proposed to explain the impact of heat on pregnancy and birth outcomes. These include heat-induced hyperthermia in early pregnancy associated with neural tube defects. Exposure to high temperatures may lead to placental insufficiency due to altered blood flow. Chronic uteroplacental insufficiency may cause fetal hypoxia and intrauterine growth retardation (IUGR) and small for gestational age (SGA) newborns. Other heat stress-related pathways include oxidative stress, inflammatory responses, and prostaglandin-mediated change. In late pregnancy, heat exposure may induce early labor and preterm birth through release of oxytocin and prostaglandins. Increased maternal core temperature can impair fetal oxygenation and growth by redistributing blood flow away from the uterus.

Study Rationale

Despite growing concern over the impact of extreme heat on maternal and fetal health, evidence on the biological pathways in humans remains sparse and inconsistent, largely due to challenges in isolating heat stress as an independent exposure during pregnancy. While animal studies and limited human experimental research have identified physiological changes under heat stress, the specific pathophysiological mechanisms linking heat exposure to adverse pregnancy and birth outcomes in humans remain poorly understood. Notably, most available evidence stems from high-income countries, despite the fact that the burden of preterm birth, stillbirth, and fetal growth restriction is disproportionately higher in low- and middle-income countries (LMICs).

This study aims to address these gaps by exploring the potential biological pathways linking heat exposure to adverse maternal and birth outcomes in a high-risk LMIC setting. In the Sindh province of Pakistan, where this study is based, pregnant women are frequently exposed to extreme temperatures both outdoors and indoors. They often engage in manual agricultural work, care for livestock, and perform household chores like cooking over open fires, often without access to cooling mechanisms. These intersecting vulnerabilities-physical exertion, prolonged heat exposure, and limited structural protection-contribute to their heightened risk.

Study Objectives

The primary objectives of the study are:

* To assess the acute and sustained effects of environmental heat exposure on pregnancy and birth outcomes in rural and peri-urban areas of Pakistan. * To assess the bio-physiological pathways through which heat stress affects maternal, fetal, and infant health across different gestational ages in rural and peri-urban areas of Pakistan.

The secondary objectives are:

• To evaluate how gestational age, socio-demographic, maternal and nutritional factors modify the association between heat exposure and adverse pregnancy outcomes in rural and peri-urban areas of Pakistan.

METHODS

Study Design and Study Sites

A prospective cohort study will be conducted among pregnant women residing in three districts of Sindh province of Pakistan: Matiari, Tando Muhammad Khan (TMK), and Tharparkar. These regions experience extreme summer temperatures and face multiple socio-economic challenges, including poor infrastructure, food insecurity, and high rates of maternal and neonatal health complications. Participants will be enrolled in their first trimester and followed through delivery and up to 12 months postpartum. The study will integrate environmental measurements, wearable sensors, imaging studies and biological samples to assess physiological, behavioral, and environmental responses to heat stress.

Sample Size

To detect a 15% increase in the risk of LBW among pregnant women exposed to heat stress compared to an unexposed cohort (1:2 ratio), and assuming 80% power, a 5% significance level, and a 20% attrition rate, 6,000 participants will be required. This calculation is based on a baseline LBW prevalence of 26.2% reported in the 2018 National Nutrition Survey for the rural population. The sample size for SVN under the same assumptions is 2,106 based on a prevalence of 46%.

For the detailed sub-study, sample size calculations were based on various key physiological parameters assessed in controlled settings, including changes in skin temperature, maternal heart rate, blood pressure, cardiac output, and fetal heart rate. To detect a 10% change in these parameters, the maximum required sample size is 200 participants, assuming 90% power, a 5% significance level, and a 20% attrition rate. However, due to the limited scope of previous studies conducted under controlled conditions and the anticipated higher attrition rate, we have chosen a sub-study sample size of 1,000 women.

Participant Recruitment and Enrollment

Pregnant women will be identified through community-based outreach in coordination with Lady Health Workers (LHWs). Field teams will conduct site mapping and community engagement to identify women who are newly married, within 1-2 years of their last pregnancy, or planning conception. Pregnancy status will be confirmed through rapid urine tests, followed by ultrasound verification conducted by study physicians at designated study offices.

After obtaining informed consent, a recruitment form will be completed. At study offices, antenatal assessment, ultrasound imaging, and biological sample collection will be performed. For the sub-study on physiological monitoring, a subset of participants will be enrolled based on their consent to wear wearable devices and allow skin temperature assessment. Transportation assistance will be provided to facilitate participation in study activities.

Data Collection Procedures

Baseline Assessments

At enrollment, gestational age will be estimated using fetal crown-rump length via ultrasound, based on the INTERGROWTH-21st protocol (±7 days accuracy). Baseline data will include sociodemographic information, medical and obstetric history, housing characteristics, occupational exposure, and anthropometric measurements. A standardized 24-hour dietary recall will be conducted at each antenatal care (ANC) visit, capturing information on food intake, frequency, meal composition, and seasonal variations.

Follow-Up Assessments

Serial ultrasounds will be conducted at 12-14, 18-22, 28-32, and 36-37 weeks' gestation, including Doppler and fetal echocardiography. Maternal blood and urine samples will be collected to assess markers such as complete blood count (CBC), hemoglobin, ferritin, glucose tolerance, dehydration status, and biological stress indicators. Samples will be handled using standardized protocols, transported in temperature-controlled containers (2-8°C), and stored at the Aga Khan University Nutrition Research Laboratory (NRL), Karachi, for analysis.

Environmental Exposure Monitoring

Ambient indoor temperature will be recorded continuously using wearable temperature/humidity loggers. Wet Bulb Globe Thermometers will provide outdoor environmental indices, accounting for multiple heat stress factors (e.g., radiant temperature, humidity, wind speed, radiation and clothing) and to calculate WBGT for each participant throughout their pregnancy.

A subset of participants will wear smartwatches to monitor heart rate, sleep patterns, and physical activity throughout pregnancy. Skin temperature will be recorded using i-Button loggers worn in customized abdominal belts. A field team will oversee device function and data transfer fortnightly.

Delivery and Postnatal Follow-Up:

At delivery, newborn weight, length, mid-upper arm circumference (MUAC), and head circumference will be recorded by trained staff within 48 hours of birth. A delivery notification system and routine calls when the participant is near term will ensure timely outcome capture.

For the sub-cohort, placental tissue and cord blood will be collected within 10-15 minutes post-delivery, snap-frozen, and stored at -80°C. Additional bio-specimens (stool and sputum) will be archived for future analyses, including epigenetic and molecular analyses. Breastmilk sampling will also be done for the sub-cohort to understand the effect of high temperatures on breastmilk quality and composition.

The mother-infant dyad will be followed for twelve months post-delivery, with monthly assessments of anthropometry, feeding practices, immunization, morbidity, and neurodevelopment.

Key maternal and newborn health outcomes will be tracked, including pregnancy complications, gestational age at delivery, birth weight, infant growth, and maternal and neonatal mortality.

Training of Study Staff and Quality Assurance

Experienced local personnel, including physicians, sonologists, nurses/ midwives, and data collectors, will be hired. Study doctors will have prior obstetrics and gynecology training, and experience in obstetric ultrasound. They will receive a 3-month training in ultrasound and maternal echocardiography at Aga Khan University (AKU), alongside completion of the International Society of Ultrasound in Obstetrics and Gynecology (ISUOG) ultrasound course.

Each study site will have three teams (two data collectors and one supervisor per team). Centralized training will include orientation to the study protocols, a question-by-question tool review, and anthropometry training per Food and Nutrition Technical Assistance III (FANTA III) guidelines. Field practice and pilot study will be conducted. Personnel operating the wearable devices will receive additional training in handling the devices and regular data retrieval.

Maternal and infant anthropometry will follow INTERGROWTH-21st standards. Maternal weight will be measured with Seca 874 U scales (precision: 0.1 kg). Newborn weight will be recorded within 48 hours using Seca 354 infant scales. Infant recumbent length will be measured with an Infantometer; head circumference and MUAC will be recorded to the nearest millimeter. All equipment will be calibrated biweekly. Maternal weight will be recorded twice by the same anthropometrist, and infant measurements will be taken independently by two blinded anthropometrists.

Statistical Analysis

Ambient heat exposure during pregnancy for each participant will be assessed using Mean Heat Index calculated from wearable temperature and humidity loggers using the Rothfusz regression equation \[67\]. Exposure will be examined across gestational windows relevant to fetal development for exploratory analyses. Descriptive statistics will summarize maternal characteristics, environmental exposures, and birth outcomes.

• Incidence of Miscarriage [Time frame: From enrollment till 22 completed weeks of gestation]
• Incidence of Pre-term birth [Time frame: At the time of delivery]
• Incidence of Small for Gestational Age (SGA) [Time frame: At the time of delivery]
• Incidence of Stillbirth [Time frame: From 22 weeks of gestation till birth]
• Incidence of Gestational Hypertension [Time frame: From 20 weeks of gestation till birth]
• Incidence of Gestational Diabetes Mellitus (GDM) [Time frame: From enrollment till birth]
• Incidence of Composite Maternal Morbidity and Mortality (CMMM) [Time frame: From enrollment till 6 weeks after birth]
• Composite adverse pregnancy outcomes for the newborn [Time frame: From enrollment till 7 days after birth]
• Incidence of Congenital Fetal Anomalies [Time frame: From enrollment till birth]