The transition of a woman to the mother’s role is one of the most important developmental procedures in the human experience. Although most women successfully develop a healthy relationship with their infants, there is still a minority that experiences some difficulties [1] .  Bonding is a complex procedure representing numerous stages in the development of the mother-infant relationship. The mother connects emotionally with the fetus, and this is a unique relationship different from any other maternal relationship as it is not usually affected by the presence of other children [2] . This prenatal attachment is linked to the formation of the cognitive representation of the fetus to which the mother attributes physical and psychological characteristics and is evident in her behavior, i.e., following a healthy diet, avoiding harmful habits such as smoking and alcohol consumption and preparing the home setting for the coming of the infant. This attachment is related to the carer’s closeness and commitment that allows a positive relationship to be created. The mother-infant bond plays a significant role not only for the infant’s well-being but also for his/her cognitive and emotional development [3] . Infants, therefore, feeling the mother’s protection and her commitment to respond to their needs and moods, develop the desire to get to know themselves, and explore the environment and the world around them through a safe attachment [4] . The mother’s response to these messages, and her reaction to the infant’s needs and mood are of paramount importance as the infant learns not only to whom and to what extent they can rely to satisfy such needs, but also qualitative characteristics are established extending to the future relations of the child with themselves and their environment. In other words, a secure or insecure attachment seems to establish some internal working models affecting the way a person regards him/herself as an infant, but also later on as a child and as an adult [5] . The child, therefore, modifies his/her behavior so as to be loved by the others, tries not to show any negative feelings, avoids creating close relations, become overly self-dependent and independent, and not express any unpleasant feelings in front of the mother, aiming at attracting her attention so as to satisfy his/her needs. In the anxious/bivalent attachment, the mother is not always available and is not sensitive towards the child’s needs, tends to ignore the infant’s signals for attention, and in general, she is unpredictable in her response leading to ignore the child’s fears and anxieties creating a feeling of abandonment to the child. This child, with this type of attachment, is unable to control his/her impulsiveness, and demonstrates negative feelings, anti-social behavior, aggressiveness, low autonomy and low social skills. The child is frequently aggressive and exerts pressure to satisfy his/her needs. Children with an insecure type of attachment believe that they are not worthy of love, that they are insignificant, do not trust their abilities, seeing the others as dismissive, unpredictable, and perhaps, punitive [3] .

With the pregnancy and the birth of a child, mothers are confronted with significant psychological and social challenges, and for some of them, these periods may entail significant emotional pain and agony. Depression affects approximately 10% of pregnant women [6] and about one in seven women in the postpartum period [7] , making postpartum depression the most widely spread clinical condition after the labor and a significant health problem. Poor mental health is one of the key risk factors for weak parent-infant interactions that can lead to adverse impacts on bonding; however, this can also occur in psychologically healthy mothers, as, for instance, when the labor conditions lead to physical separation and the early physical contact between the mother and the neonate is prevented which is of decisive importance for the establishment and consolidation of their relationship [8] . The mother’s response to the infant’s needs, her closeness and availability as the main carer are affected both by her mental condition, as in the cases when postpartum depression is triggered when there is a non-treated mental disease, but also in mentally healthy mothers by the mother’s age, the pregnancy characteristics, the type of labor, possible perinatal loss, the mother’s primiparity or multiparity, the socioeconomic status of the family, the changes in the structure and composition of the traditional family that have resulted in the mother playing multiple roles, not only that of the parent-carer, but also of the working woman, and finally by external environmental factors. Pregnant and postpartum women face unique challenges in the context of the COVID-19 pandemic and may be at increased risk for mental health problems in this setting [8] .

Τhe mother-infant bond is affected by the support the mother will have or require or need to have in periods of crisis. A relevant instance was the COVID-19 pandemic caused by the SARS-CoV-2 virus. More specifically, the COVID-19 pandemic has led to numerous social, economic, and psychological shifts that have impacted the emotional well-being of families, creating a challenging environment for parenting [9] . Research during the pandemic, has indicated a rise in parenting stress, anxiety, fear, relational conflicts, sleep disturbances, and postpartum depressive symptoms, all of which can negatively affect the parent-child bond [10] . These findings have highlighted concerns about the pandemic’s effects on child development. Given that COVID-19 pandemic has affected the psycho-social life of all people, it is essential to investigate the effect of the COVID-19 on the mother-infant bond. Thus, the aim of this study is to investigate the impact of a stressful environmental factor, the COVID-19 pandemic, on the mother-infant bond, through the systematic review of the literature.

Notes: Edinburgh Postnatal Scale (EPDS); Generalized Anxiety Disorder-7 (GAD-7); Postpartum Bonding Questionnaire (PBQ); Prenatal Attachment Inventory (PAI); Depression, Anxiety and Stress Scale (DASS); Impact of Event Scale-Revised (IES-R); Dyadic Adjustment Scale; (DAS-16); Perinatal Posttraumatic Stress Disorder Questionnaire (PPQ); Mother-Infant Bonding Scale (MIBS); Center for Epidemiologic Studies-Depression (CES-D); Maternal Self-Efficacy Scales; Perceived Social Support (MSPSS); Self-compassion (SCS-SF); Mindful parenting (IM-P—Infant version); Parental Stress Scale (PSS); Postnatal Sense of Security Instrument (PPSSi); Hospital Anxiety and Depression Scale (HADS); state-trait anxiety inventory (STAI-YA); coronavirus health impact survey questionnaire (CRISIS); Peritraumatic Distress Inventory (PDI); Posttraumatic Checklist for DSM-5 (PCL-5); Life Events Checklist for DSM-5 (LEC-5); Corona disease anxiety scale (CDAS); Müller Maternal Attachment index (MAI); Parenting sense of competence scale (PSOC); COVID-19-related resource worries COVID-19; Connor-Davidson Resilience Scale; Distress Tolerance Scale; The Life Events measure.

This systematic review included 18 articles. More specifically, 12 cross-sectional studies, 5 prospective and cohort studies and a randomized controlled trial. The total number of women was 12.452 with data been collected via self-completed questionnaires. All papers investigated the exposure of pregnant or postpartum women in the COVID-19 period and the kind of mother-infant quality of attachment as an outcome. The total sample contained data from Canada [12] , USA [13] - [17] , Portugal [18] [19] , Italy [20] , France [21] - [23] , Japan [24] , Iran [25] ,  Lebanon [26] , Turkey [27] , Israel [28] and Kazakhstan [29] ( Table 1 ). Differences in the methodological quality and risk of bias of the following articles were insignificant (Quality assessment column of Table 1 ). The quality assessment of the studies was evaluated through deferments tools. The cross-sectional studies were evaluated with the Centre for Evidence-Based Medicine Critical Appraisal (CEBM) [30] , the cohort and prospective studies with the Newcastle-Ottawa scale (NOS) [31] and the randomized controlled trial with the CONSORT [32] . The results show that the risk of bias remains medium to low ( Table 1 ).

According to our findings, mothers during the COVID-19 period, suffered from mental health issues and as a result from a reduced mother-infant bonding. It is well known, that the COVID-19 period affected the mental health of the overall population [33] , especially, the women during the perinatal period [34] . It seems that the incomplete knowledge about the evolution of the pandemic and the risk of loss of life, are factors in the development of post-traumatic symptoms [35] . Of course, few of our studies examined post-traumatic stress; however, the close relationship between anxiety, depression and post-traumatic stress [36] , is well known in the literature, so we believe that women faced these posttraumatic symptoms. Also, the risk of losing the infant’s life is considered by the mother to be a greater psychological trauma than the risk of losing her own life [37] , which indicates the extent of mothers’ stress at that time. On the other side, mother-infant bond, seems to be affected from mental health status [38] [39] of the mother. The relationship between low levels of maternal attachment and higher anxiety and depression or PTSD is consistent with the existing literature [38] [40] . It seems that social changes during the pandemic increased the mother’s vulnerability to stressful situations, affecting the process of creating an emotional bond with the infant. Both the literature and empirical data suggest that internalized negative emotion, such as stress and postpartum depression, can hinder a mother’s ability to bond with her infant [41] . On the contrary, the strong mother-infant bond reduces the negative effects of the depressive and anxiety symptoms of postpartum mothers [42] , and for this reason, land support is imperative.

In conclusion, given the importance of the healthy development of the mother-infant bond, in the mental development of the child and in improving or strengthening the mental health of the mother, our findings indicate the need for intervention by midwives and specialist perinatal professionals in difficult epidemiological periods, strengthening the maternal bond and preventing psychosocial disorders in the mother-child pair. We recommend more research on the mother-infant bond, not only to investigate the current situation due to the pandemic, but to be able to deal with similar situations in the future, such as pandemics or situations that affect the mother’s mental health.

Conceptualization: M.A. and E.A.; methodology: M.A.; validation: E.O. and E.A.; investigation: M.A. resources: M.A. and E.O., data curation: M.A., writing—original draft preparation: M.A., writing—review and editing: M.D. and M.I., visualization: M.A.; supervision: E.A. All authors have read and agreed to the published version of the manuscript.