The Bottom Line

This pages serves to highlight the latest objective research, news, and professional guidance and recommendations regarding pregnancy-related health.

As of September 2021, COVID-19 and heat stress are the current major health-related issues that could adversely affect pregnant women. Women should also take note that Respiratory Syncytial Virus (RSV) cases are beginning to increase in the U.S. Although RSV is a minor infection among adults, researchers recommend pregnant women ask to be tested for this common virus if other diagnoses are ruled out.

This page will highlight COVID-19 infection and vaccine-safety data as it is published/released, as well as evidence-based information on how women can avoid heat-related effects during pregnancy.

Additional highlights and research updates on other topics are also listed below. Women should talk to their health care provider (HCP) if they have any questions.

Note: Unless ongoing, highlights are kept on this page for approximately 30 days before being removed or added to a permanent Topic page.

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Latest Page Updates

Pages updated within the past week (as of September 12, 2021):

Photo by ARTO SURAJ on Unsplash

Latest Research/Recommendations

September 7, 2021: Study assessed Pfizer COVID-19 vaccine effectiveness during pregnancy in 10,861 vaccinated participants and 10,861 unvaccinated matched controls. Read more here.

September 3, 2021: A retrospective cohort study of 1,918,105 fetuses between 2 and 8 weeks after gestation from May to October found that extreme heat events (90th or 95th percentile of the daily maximum temperature) increased the risk for congenital heart defects (see Heat Exposure for more information).

September 2, 2021: Study found that early vs. late third trimester COVID-19 mRNA vaccination appears to “maximize transplacental antibody transfer and neonatal neutralizing antibody levels.” This indicates that getting vaccinated in the early third trimester may offer higher transfer of COIVD-19 antibodies to the fetus via the placenta.

September 2, 2021: United Kingdom (UK) Yellow card data indicate there is no current evidence that COVID-19 vaccination while breastfeeding causes any harm to breastfed children or affects the ability to breastfeed. Learn more about this report here.

August 25, 2021: Very small review analyzed data from four COVID-19 positive pregnant women who received monoclonal antibody treatment. While all four women had underlying conditions, the review noted there were no complications reported at time of publication and all four women recovered. The authors noted that "given the ongoing severity of the COVID-19 pandemic, especially in pregnancy, more information regarding the safety and efficacy of neutralizing [monoclonal antibody treatment] in pregnancy is vital."

August 20, 2021: Study demonstrated that the presence of depressive and anxiety symptoms prior to labor increased stress and pain perception during labor. The authors indicated these findings highlight the need to provide high risk women with emotional support as well as pain relief options to alleviate stress during labor.

August 17, 2021: Study published short-term reactions among pregnant and lactating individuals in the first wave of the COVID-19 vaccine rollout:

  • 7809 individuals who were pregnant; 6815 individuals who were lactating; 2901 individuals who were neither pregnant nor lactating but planning pregnancy in the near future

  • Most common reactions were pain at injection site and fatigue

  • Frequency of reactions after the second dose was higher than after the first dose

  • Average maximum temperature was 100.6 °F/ 38.1 °C after dose 1 and 100.7 °F/38.2 °C after dose 2 (including 1051 pregnant individuals)

  • Decreased milk supply for less than 24 hours was reported by 339 lactating individuals after the first dose (5.0%) and 434 individuals after the second dose (7.2%)

August 16, 2021: United Kingdom reporting on COVID-19 vaccine safety during pregnancy from Yellow Card data (the Yellow Card program is the UK system for collecting and monitoring information on safety concerns such as suspected side effects or adverse incidents involving medicines and medical devices). Read findings from this report here.

August 16, 2021: CDC is currently investigating or recently investigated salmonella outbreaks due to backyard poultry (chickens, chicks, ducklings) as well as pet hedgehogs and pet turtles. Pregnant individuals should use proper hygienic techniques and other methods to avoid getting sick when handling these animals. Learn more.

August 12, 2021: Among 906,319 eligible women in Ontario with a Gestational Diabetes Mellitus (GDM) diagnosis with a live birth delivery between 1 July 2007 and 31 March 2018, there were 763 heart failure events over a median follow-up period of 7 years. GDM also increased the odds of peripartum cardiomyopathy (weakening of the heart muscle near delivery and through five months postpartum). Women diagnosed with GDM during pregnancy should receive proper cardiovascular follow up in their postpartum months/years.


Heat Exposure

Hyperthermia (opposite of hypothermia, which occurs from “cold” exposure) is emerging as a risk factor for potential negative outcomes regarding fetal development. 

Current evidence indicates that prolonged exposure to heat can raise the body’s core temperature and potentially induce harmful effects on the fetus, especially in the first trimester.  However, “prolonged” exposure is not defined, and the minimum core body temperature at which damage may begin is not known for certain (currently assessed to begin at a body temperature of 102° F/38.9° C).

Any source of heat that can raise core body temperature during pregnancy for a prolonged period may lead to adverse pregnancy outcomes, depending on length and frequency of exposure.

Pregnant women are advised to:

  • Avoid prolonged exposure to hot weather

  • Remain in air conditioned rooms when possible

  • Stay hydrated

  • Protect the skin with sunscreen, light clothing, and hats

  • Avoid exercising in hot weather and hot environments (e.g. hot yoga); see our Exercise page for more information

  • Replace sweat lost with additional fluids, to include fluids with electrolytes if necessary

  • Keep shower and bath length (when using hot water) to 10 minutes or less (to be cautious)

  • Learn to recognize the signs and symptoms of heat stroke and exhaustion in order to ask for help when necessary

Learn much more detail about Heat Exposure here.

Recent Heat-Related Research:

A study published in September 2021 of 1,918,105 fetuses between 2 and 8 weeks after gestation from May to October found that extreme heat events (90th or 95th percentile of the daily maximum temperature) increased the risk for congenital heart defects (see Heat Exposure for more information).

Of 1,918,105 infants, 9588 had CHD, with a prevalence rate of 499.9 per 100,000. The authors found that extreme heat events during 2-8 weeks postconception was associated with overall CHD in offspring, particularly atrial septal defects and patent ductus arteriosus. The associations strengthened with the extent and cumulative days of maternal exposure to EHE.

A study published in August 2021 used birth records for 315,226 infants born in Queensland, Australia and matched to average maximum and minimum temperature for the last month of pregnancy. Higher temperatures were associated with small increases in the odds of delivering an infant in the extreme preterm period. This risk was further increased for women who smoked during pregnancy.

A study published in July 2021 reviewed all emergency department (ED) visits and hospital admissions due to pregnancy complications from 2005 to 2013 in New York State. Daily mean temperature > 90th percentile of the monthly mean temperature in each county was defined as an Extreme Heat Exposure (EHE).

The study determined EHE was significantly associated with increased ED visits for pregnancy complications in summer. There was also a significant and stronger association in transitional months (May and September). The authors noted that "earlier warnings regarding extreme heat are recommended to decrease pregnancy complications."

A study published in June 2021 provides evidence that individuals in their second and third trimester of pregnancy are not at greater risk for hyperthermia during moderate exercise in the heat (45 minutes, 32 °C/89.6 °F). Although pregnant women felt hotter, their body temperatures and sweat output were not different than the control group; core temperature of all pregnant participants never exceeded 38 °C/100.4 °F.

Of note, however, it is possible that higher intensity exercise, or shorter bouts of high intensity exercises in a climate above 32 °C/89.6 °F (or with higher humidity), could raise body temperature higher than observed in this study. Further, this study size was small and body temperature can vary widely among different individuals.

A study published in April 2020 suggested that acute exposure to extreme heat during the last week of gestation may adversely trigger an earlier delivery. Approximately 7% of the cohort had a preterm birth. For all definitions of extreme heat, the risk of preterm birth was consistently higher among mothers who experienced an extreme heat episode during their last week of gestation.


COVID-19 and Vaccines

Background: In March 2020, the World Health Organization officially declared a worldwide pandemic of a new respiratory infection called coronavirus 2019 (COVID-19), more formally “severe acute respiratory syndrome coronavirus 2” or SARS-CoV-2.

As of September 16, 2021, the U.S. Centers for Disease Control and Prevention (CDC) estimates the virus has caused approximately 41,593,179 illnesses and 666,440 deaths.

States with the highest case rates per 100,000 people (as of September 16): Alaska, South Carolina, Tennessee, West Virginia, and Wyoming.

General COVID-19 information and pregnancy health: According to currently available case series and cohorts as of September 2021, data indicates pregnant women are not more susceptible to the disease than non-pregnant women, and fetal and symptomatic neonatal infection appear to be rare events.

Weekly COVID-19 Pregnancy Data from CDC:

  • Total Cases: 120,459

  • Hospitalized: 21,387 (data only available for 95,640)

  • Total Deaths: 155

*As of September 16, 2021

These numbers likely do not include all pregnant women with COVID-19 in the United States and must be interpreted with caution.

However, pregnant women who contract COVID-19 appear to be at higher risk of complications from the disease than non-pregnant women of reproductive age. These complications include Intensive Care Unit admission, the need for mechanical ventilation, and the need for extracorporeal membrane oxygenation (ECMO). Learn more.

There are currently three COVID-19 vaccines in the United States that have been given Emergency Use Authorization by the FDA: Pfizer-BioNTech, Moderna, and Johnson & Johnson/Janssen.

Photo by Mufid Majnun on Unsplash

The FDA does not have a specific recommendation regarding whether one vaccine is more preferable over another, to include during pregnancy and lactation. It is recommended by numerous expert panels and obstetric organizations that women choose for themselves whether they would like to be vaccinated, after consultation with their HCP.

In the U.K., the Pfizer-BioNTech, Moderna, and Vaxzevria (formerly Oxford-AstraZeneca) vaccines are currently authorized by its vaccine regulator, the Medicines and Healthcare products Regulatory Agency.

Read more on COVID-19, pregnancy, and more detailed vaccine information and guidance from the American College of Obstetricians and Gynecologists, the Society for Maternal-Fetal Medicine, the Society of Obstetricians and Gynecologists' of Canada, the U.K. Royal College of Obstetricians and Gynaecologists, and The Royal Australian and New Zealand College of Obstetricians and Gynaecologists here.


Women should talk to their HCP if they any questions related to any of the updates above. Not all pregnant women who contract COVID-19 will have serious illness. However, there is currently no way to determine which women may experience a more severe course of the infection.

It is recommended that pregnant women:

  • Seek care immediately or dial 911 if experiencing a medical emergency. Emergency symptoms can include trouble breathing, persistent pain or pressure in the chest, new confusion, inability to wake or stay awake, or bluish lips or face. This is not an inclusive list, and women should always call their HCP or visit an emergency room when necessary.

  • Report any and all symptoms of a respiratory infection to their HCP, to include cough, fevershortness of breath, and diarrhea

  • Avoid unnecessary traveling, use of public transportation, and contact with sick people

  • Wear a mask (to protect others) in public settings

  • Follow personal and social distancing rules

  • Regularly wash hands for at least 20 seconds (if soap and water are not readily available, use a hand sanitizer that contains at least 60% alcohol)

  • Avoid touching their eyes, nose, and mouth with unwashed hands.

  • Do not skip prenatal care appointments or other health appointments that may be necessary

  • Make sure to have at least a 30-day supply of prescription medications

  • Learn about stress and coping

  • Get adequate sleep and physical exercise

  • Quit smoking (risk factor for infection)

  • Go outside in open areas for walks, fresh air, and a change of scenery

Any pregnant woman who has traveled in a state or country with a high rate of COVID-19 infection or who has had close contact with an individual with confirmed infection should be tested and quarantined.

Pregnant women are also advised to see their HCP immediately or call 911 if they experience:

  • Difficulty breathing or shortness of breath

  • Persistent pain or pressure in the chest or abdomen

  • Persistent dizziness, confusion, inability to arouse

  • Seizures

  • Not urinating

  • Severe muscle pain and/or weakness

  • Fever or cough that improves but then returns or worsens (may indicate pneumonia)

  • High fever that is not responding to acetaminophen or other anti-fever medication (acetaminophen is not an NSAID)

Women should refer to the CDC and ACOG COVID-19 web pages below for additional information.


Total cases in the United States/COVID tracker (U.S. Centers for Disease Control and Prevention)

COVID-19 Home Page (U.S. Centers for Disease Control and Prevention)

ACOG Statement on COVID-19 and Pregnancy (American College of Obstetricians and Gynecologists; June 2020)

Coronavirus (COVID-19), Pregnancy, and Breastfeeding: A Message for Patients (American College of Obstetricians and Gynecologists)

The Management of Respiratory Infections During Pregnancy (Immunology and Allergy Clinics of North America; 2006)


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