Is the Course of COVID-19 Different during Pregnancy? A Retrospective Comparative Study

The COVID-19 pandemic has challenged health systems around the world. Maternal-foetal medicine, which has been particularly affected, must consider scientific data on the physiological processes occurring in the pregnant woman’s body to develop relevant standards of care. Our study retrospectively compared the clinical and laboratory characteristics of 52 COVID-19 pregnant patients with 53 controls. Most of the pregnant patients required medical attention during the third trimester and therefore we propose that vaccination is needed prior to the 30th week of pregnancy. We found no differences between the 2 groups in the course of illness classification system, days of hospital stay, need for oxygen supplementation, need for mechanical ventilation, and ICU admission. Moreover, clinical manifestations and imaging findings were comparable. Pregnant patients needed a greater oxygen flow rate and required high flow oxygen therapy more frequently. Considering pregnancy-related physiological adaptations, we found that COVID-19 infection in pregnant patients is associated with higher levels of inflammatory markers, apart from serum ferritin, than in non-pregnant women, and concluded that biomarkers of cardiac and muscle injury, as well as kidney function, may not be good predictors of COVID-19 clinical course in pregnant patients at the time of admission, but more research needs to be conducted on this topic.


Introduction
Coronavirus disease 2019 , caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), was declared a pandemic by WHO on 11 March 2020, and has exposed vulnerable populations to an unprecedented global health crisis. Maternalfoetal medicine was unprepared and struggled to mount an adequate response. As new data on SARS-CoV-2 are emerging, guidelines regarding perinatal care are constantly being updated to balance evidence-based maternity care with COVID-19 management and treatment strategies.
The SARS-CoV-2 infection mainly affects the respiratory system, causing mild or moderate respiratory symptoms in 85% of cases. Cardiovascular, renal, neurological, psychiatric, dermatological, and gastrointestinal manifestations have also been reported [1][2][3][4][5]. Guan et al., [2] reported fever (88.7%), cough (67.8%), fatigue (38%), sputum production diagnosis of COVID-19. The outcomes we measured in relation to the severity of the disease were: course of illness classification system, increased demand for oxygen supplementation, days of hospital stay, need for oxygen supplementation and high flow oxygen therapy, need for mechanical ventilation, and ICU admissions.

Study Population
This retrospective single-centered study was carried out in the Department of Obstetrics and Gynecology, at the Central Clinical Hospital of the Ministry of the Interior and Administration in Warsaw, Poland. The control group was comprised of non-pregnant women of reproductive age who were randomly recruited from those designated for COVID-19 treatment in Hospital Departments. From 15 May 2020 to 26 April 2021, a total of 52 pregnant and 53 non-pregnant women with COVID-19 infection were admitted for treatment. The research was approved by the Bioethics Committee of the Central Clinical Hospital of Interior and Administration in Warsaw.

Inclusion and Exclusion Criteria
Inclusion criteria were temperature >39 • C despite the use of paracetamol, tachypnoea >30/min, SaO 2 <95% on room air, oxygen requirement, or critical course of illness. The COVID-19 diagnosis was confirmed by positive RT-PCR assay performed within no more than 13 days prior to admission.
Exclusion criteria were hospital admissions for problems other than those reflecting a deteriorating condition.

Clinical Course of Illness
We divided the patients into 4 groups based on the Polish Association of Epidemiologists and Infectiologists [33] classification guidelines determined by the severity of symptoms and the results of tests. Mild (asymptomatic, or the presence of cough, fever, dyspnoea, fatigue, headache, muscle pains, nausea, vomiting, diarrhoea), moderate (clinical and radiological features of lung occupation), severe (respiratory failure, low peripheral SpO 2 < 90%), or critical (ARDS, hypotensive shock, multiorgan failure, loss of consciousness).

Study Procedures
At admission, all the women underwent blood, urine, coagulation, and biochemical blood tests, as well as computed tomography (without contrast) in cases where moderate, severe, or critical forms of COVID-19 were suspected.
The following variables were analysed: patient age, weight, height, body mass index (BMI), pre-existing medical conditions (diabetes mellitus, hypertension, hypothyroidism, asthma), symptoms, physical examination results, pregnancy status, and gestational age at the initial presentation.

Statistical Analysis
Statistical analysis was performed using Statistica 13.1 statistical software (StatSoft Polska Sp. z o.o., Kraków, Poland). Mean values and standard deviations were used to describe the study groups; in case of skewed distributions the median was calculated as a measure of central tendency, and the scatter was presented using the 25th and 75th percentile of the distribution; in case of qualitative variables the data were presented as a percentage. After assessing the normality of the distributions of the explanatory variables using the Shapiro-Wilk test, a comparative analysis of each group was performed. For variables that did not meet the assumption of near normal distribution, the Mann-Whitney U test was used to compare groups; for variables that met the assumption of normal distribution, the Student's t-test was used for comparison. In the case of qualitative variables, the chi-squared test was used to compare the frequencies of the studied characteristics in groups. Fisher's exact test was applied when the expected number of frequencies in the chi-square test was less than 5. Differences were considered statistically significant at p < 0.05.

Patients' Characteristics and Comorbidities
Mean gestational age at admittance was 30 weeks. Based on current knowledge, a severe and critical COVID-19 course of illness is mainly a hyperinflammatory, immunemediated disorder, triggered by a viral infection. As a result of their immunological features, pregnant women are supposed to be particularly susceptible to intracellular infections as well as immunological disturbances, especially during the late second and third trimesters [27]. During the second trimester, these changes are characterized mostly by the elevation of humoral immune responses and suppression of cell-mediated immunity, referred to as the T-helper lymphocyte type 1-type 2 (Th1-Th2) shift [34,35]. Therefore, Th1 cell-mediated immunity is compromised, increasing the susceptibility of pregnant women to viral and intracellular bacterial infections [26,27,36]. During the third trimester, increased numbers of monocytes and granulocytes are found in maternal blood, compared to non-pregnant women, releasing inflammatory cytokines, e.g., IL-8, TNF-a, and IL-6 [27,37,38]. Moreover, pathophysiological processes responsible for the development of hypertension, diabetes, or cardiovascular diseases intensify during the 3rd trimester and pregnant women with comorbidities and older pregnant women appear to be at particularly elevated risk of adverse maternal outcomes [23,[39][40][41][42]. We believe that this should addressed by implementing preventive measures in the form of vaccination prior to week 30 of pregnancy. The use of mRNA vaccines for both maternal and foetal benefits is well established in the literature. The efficiency and safety of inoculation against SARS-CoV-2 in pregnant and lactating cohorts were underlined in studies by Collier et al., and Gray et al. [43,44]. Antibodies detected in umbilical blood and milk suggests the possibility of infant protection and their cord blood titers show a correlation with the time from vaccination to delivery [45]. Data show the presence of foetal antibodies after 15 days of both mRNA vaccine and infection. [46] However, impaired placental transmission during the third trimester has been recorded [46,47]. Therefore, late second trimester may be proposed as a perfect time for vaccination [48], although further evaluation of this is needed.
Hypothyroidism was the only comorbidity that was statistically more frequent in the pregnant group. Thyroid hormone dysfunction could affect maternal COVID-19 outcomes and increase mortality in patients with a critical illness [49][50][51]. Moreover, in a retrospective study of hospitalized moderate-to-critical COVID-19 patients, 56% of the 50 subjects studied showed lower-than-normal TSH values during their infection [52]; it has been suggested that it is necessary to monitor thyroid hormones in COVID-19 patients [53]. In our opinion however, serum TSH FT3 and FT4 concentrations in pregnant patients with a combination of primary hypothyroidism and levothyroxine treatment might be misinterpreted. Our two groups did not differ in terms of the time of SARS-CoV-2 onset, length of hospital stay, and lung involvement in the CT scan. In contrast, more severe disease at imaging has been documented in a cohort of pregnant patients from China [54].

Symptoms
The high prevalence of dyspnoea (n = 48, 92.31%), cough (n = 47, 90.38%), fever (n = 33, 63.46%), fatigue (n = 22, 42.31%), and muscle aches (n = 22, 42.31%), and the greater frequency of dyspnoea in pregnant group may be partially associated with the physiology of pregnancy and is consistent with the findings of Zambrano et al., [39]. Other studies among pregnant patients have revealed 73% [41] and 95% [55] were asymptomatic. Pregnant women have also been found to be more likely to be asymptomatic than nonpregnant women of reproductive age with COVID-19 [41]. However, these analyses included pregnant and recently pregnant women diagnosed as having either suspected or confirmed COVID-19 who were attending or admitted to hospital for any reason. At any given time, pregnant women account for 3% (234 million) of the world population of 7.8 billion [56] and only fraction of this cohort was tested. Therefore, it is not possible to identify with complete accuracy how many pregnant women have been affected and all pregnant persons should be monitored for development of symptoms and signs of COVID-19.

Clinical Course Comparison
Our case-control study, which included patients admitted to hospital for non-obstetric reasons with RT-PCR confirmed infection, indicated that pregnant women diagnosed with COVID-19 did not present more severe outcomes than their nonpregnant counterparts. Data emerging from the meta-analyses by Allotey et al., [41] Khali et al., [57] and Mazur-Bialy et al., [58] show that pregnant women may have an increased risk of developing severe symptoms. In meta-analysis made by Zambrano et al., [39] that included over 23,000 pregnant women and more than 386,000 nonpregnant women of reproductive age with symptomatic laboratory-confirmed SARS-CoV-2 infection, pregnant patients had a higher risk of ICU admission, receiving invasive ventilation, ECMO, and death. However, lack of information on the women's pregnancy status and reasons for hos-pitalisation limits the usefulness of the study's findings. Another meta-analysis made by Martinez-Portilla et al., [59] that compared 5183 pregnant with 175,905 nonpregnant women after propensity score-matching revealed a higher risk of death, pneumonia, and ICU admission.
Our study showed that the oxygen flow rate and the need for high-flow oxygen therapy were both higher in the pregnant group in comparison with the non-pregnant subjects. Due to a comparable lung involvement percentage in the 2 groups, the physiological adaptations during pregnancy (higher diaphragm position due to increased uterus size, increased oxygen demand, and progesterone-induced airway oedema) may play a role.

Laboratory Findings in Comparison with Non-Pregnant Group
The results of the laboratory tests performed on the pregnant group on admission comparing the inflammatory markers with those non-pregnant group revealed that the leucocyte counts, neutrophiles counts, CRP levels, procalcitonin levels, and IL-6 were higher, the exception being ferritin levels. Some findings are consistent with similar studies and may be explained by the hyper induction of the immune system caused by pregnancy and the SARS-CoV-2 infection [60]. Data from Liu et al., and Qiancheg et al., shows that an elevated leukocytes count in the blood is significantly more frequently observed among pregnant COVID-19-affected women [61,62]. Nevertheless, an analysis of the data reported by Liu et al., shows that the number of leukocytes is almost in the high normal range of values for pregnant women. Additionally, mild leukocytosis physiologically occurs in the third trimester [61][62][63]. Serum levels of ferritin were not significantly higher, probably due to the higher frequency of iron deficiency anaemia in the pregnant group [64].
In our study, the hypercoagulation state manifested in higher D-dimer and fibrinogen levels and lower prothrombin times may have been partially pregnancy-induced. Available data suggest an increased risk of venous thromboembolism (VTE) related to COVID-19 in infected pregnant patients compared with uninfected pregnant patients [65][66][67].
Our study revealed lower haemoglobin, platelets, creatinine, and urea levels typical for pregnancy resulting from hemodilution, increased renal flow rate and pregnancy-related thrombocytopaenia.
We also detected some shifts in ion levels due to the physiological changes during pregnancy, namely lower magnesium, calcium, and sodium levels.
Interestingly, we also discovered significantly lower NT-proBNP levels in the pregnant group and similarly for troponin levels. Nt-ProBNP is produced by the heart and the pregnant uterus, because of relevant cardiac volume overload in chronic heart failure. Between 8 and 28% of COVID-19 patients show evidence of cardiac injury with elevated troponin and natriuretic peptides [68,69]. Some data support NT-proBNP as an independent risk factor for in-hospital death, ICU admission, mechanical ventilation, and coagulopathy in patients with severe COVID-19 disease [70].

Conclusions
Most pregnant patients infected with SARS-CoV-2 require medical attention during the third trimester. We propose that all pregnant women should be vaccinated prior to the 30th week of pregnancy.
Pregnant women diagnosed with COVID-19 did not present more severe outcomes than their non-pregnant counterparts, probably due to the small patient sample and imperfect measuring methods. Clinical manifestations of COVID-19 disease were similar in the two cohorts, and some, like dyspnoea or fatigue, overlap with the symptoms of normal pregnancy.
The laboratory and imaging findings were generally similar in both the pregnant and non-pregnant groups. Laboratory results for the coagulation test, morphology, or serum levels of ions can be explained by normal pregnancy-related physiological processes. Moderate and severe COVID-19 clinical courses in pregnant women are associated with higher levels of inflammatory markers, apart from serum ferritin, than in non-pregnant women, which might be partially explained by the imposition of immunological features and iron deficiency anaemia. Biomarkers of cardiac and muscle injury, as well as kidney function, may not be good predictors of COVID-19 clinical course in pregnant patients at the time of admission, but more research needs to be conducted on this topic.

Research Implications
Future studies of pregnant patients infected with SARS-CoV-2 should focus on the assessment of disease severity predictors for early identification of patients at risk of developing complications, and thus improve optimalization and preventative efforts in this cohort. Further validation with a larger study group and risk models are needed to provide useful data for the effective management of health resources in the COVID-19 era.

Strengths and Limitations
The main strength of our study is the variety of parameters evaluated. The main weaknesses of our investigation include its single-centre nature, as well as a small and homogeneous group of patients.