1. Context
nfantile Colic (IC) is a prevalent disease in early infancy that can present significant adverse effects on infants and their families (1). IC refers to uncontrollable crying for more than 3 hours a day, 3 days per week, and 3 consecutive weeks (2). This behavioral phenomenon (i.e. crying & restlessness) usually manifests at the age of 4-16 weeks with the peak of incidence in 6 weeks of age. The prevalence of IC was reported to be 15%-30%. Besides, it is more common in families experiencing childcare for the first time (3, 4, 5). They not only have periods of abdominal pain, excessive crying, and insomnia in early infancy but also may experience behavioral and mood disorders, hyperactivity, and migraine at older ages (6, 7). Additionally, uncontrollable crying causes stress and anxiety in parents and can even impose the risk of abuse on the infant (8). Despite extensive research, the main cause of IC remains unknown; various factors in the infant, such as insufficient neurodevelopment, Gastrointestinal Tract (GIT) disease (the inflammation & imbalance of intestinal flora), and the psychosocial problems of parents have been associated with IC (9, 10). A closer look at the classification of IC risk factors reveals that a number of them are related to infants and others to parents. Parental characteristics, such psychological status, and maternal diet were assessed in different studies with controversy in the obtained data (11, 12, 13). 
Identifying parental risk factors is of high significance; a number of these characteristics can be prevented by education and require no pharmacotherapy. Therefore, these factors can reduce or eliminate with precise planning even during pregnancy. For example, the swallowing of air due to improper breastfeeding techniques exacerbates IC symptoms. Accordingly, this issue can be relieved through teaching the correct techniques by the medical staff. However, infant-related factors, such as the conditions of intestinal microbial flora require probiotic (containing beneficial bacteria) administration (14, 15).
Recent review studies on IC revealed that researchers put more emphasis on treatment, especially using probiotics, and pay less attention to the role of the parental factors on IC. Our search also leads to no comprehensive review study that assessed different parental risk factors on IC incidence. Therefore, conducting a review study focusing on parental risk factors that systematically collects information from reliable scientific databases was necessary.
The present study was conducted to investigate related parental risk factors affecting IC incidence. The Population, Intervention, Comparison, and Outcome (PICO) criterion was used to clarify the goal and select appropriate studies for this review. Accordingly, the study population was parents (mother, father, or both) of infants with colic. IC was defined based on Wesel (the rule of three), Rome III and IV criteria, or referrals to physicians due to excessive crying and the lack of other physical problems. The intervention was not applicable for this review study and only observational studies were evaluated. Furthermore, comparisons were conducted between the parents of infants with colic and controls in prospective and case-control as well as cross-sectional studies (if there was a control group). The relevant outcome was considered the studies that have examined parental factors on IC incidence.

2. Evidence Acquisition 
We searched major databases, such as PubMed, Web of Science, Scopus, Science Direct, Google Scholar (the first 100 articles), as well as Scientific Information Database (SID) and Magiran (Iranian websites) after selecting the relevant keywords based on Medical Subject Heading (MESH). For example, we used the following search strategy in PubMed database: “((parents(MeSH Terms)) OR (mothers(MeSH Terms))) OR (fathers(MeSH Terms)) AND (causality(MeSH Terms)) OR (risk factors(MeSH Terms)) AND colic(MeSH Terms) AND infant(MeSH Terms)”.
The PRISMA checklist was applied to retrieve the final articles (Figure 1).

The inclusion criteria were as follows: Observational studies (case-control, prospective, & cross-sectional); English or Persian language articles; IC was defined based on Wessel’s criteria and Rome III and IV criteria, or visiting a physician due to the excessive crying of the infant; parental factors affecting IC, i.e. discussed and separately presented in study results, and the availability of the full text of the articles. The exclusion criteria included experimental, letter to the editor, and review studies. No time limitation was considered for searching in databases. The systematic review registration number in the International Prospective Registration of Systematic Reviews (PROSPERO) was CRD42020163518.
A total of 423 articles were selected from the databases and transferred to the Endnote. Next, duplicate (n=21) and non-relevant articles were removed (n=350). The abstracts of the remaining articles were reviewed in terms of study design and the availability of full text, where 16 articles were omitted in this step. The full texts of the selected articles were assessed according to the clarification of IC definition, parental results, and quality assessment by two researchers; subsequently, 23 articles were excluded (Figure 1). Finally, the data of 18 papers, including the names of the authors, year, country, the purpose of the study, the number of parents participating in the study, and the main results were recorded in Table 1.


Additionally, the main detected risk factors were listed in Table 2.

The compliance of studies with the inclusion and exclusion criteria was evaluated by two researchers (MSc in nursing) and consulting with a statistician. Besides, the quality of articles was checked by the STROBE checklist (Table 3).

This checklist includes 22 items and is suggested for the quality promotion of observational studies. Some items of this checklist are study design, sample selection, data collection, analysis, and potential bias (16). The discrepancy for each item was resolved through a scientific discussion between two researchers. Papers that received scores above the average were included for data extraction. No articles were not excluded due to quality appraisal.

3. Results
Eighteen articles with a total sample size of 13523 (infants’ parents) were selected; accordingly, their results were extracted, including 10 prospective, 6 cross-sectional, and 2 case-control studies. The databases were searched for articles published at any time up to the end of December 2019. Among the studies, 5(27.77%) were from Turkey, 3(16.66%) from Sweden, 3(16.66%) from the United States, 3(16.66%) from the Netherlands, 2(11.11%) from Denmark, and 2(11.11%) from Iran. The data of the conducted studies are summarized in Table 1 based on the objectives, sample size, and results. Eventually, the effective factors were placed in 6 categories, as follows: psychological factors; physical factors; perinatal factors; family socioeconomic status; the type of diet, and taking medications. These factors are described in detail in the discussion section (Table 2).

4. Discussion
The present systematic review examined the parental factors influencing IC incidence. To facilitate the description of the results, we divided them into 6 categories, as follows: psychological, physical, and perinatal factors, family socioeconomic status, maternal diet, and taking medications, i.e. discussed below.
In this category, studies have mostly focused on maternal depression and anxiety during pregnancy and after childbirth (13, 17, 18, 19, 20). Both of them are considered destructive factors in mother-infant interaction and even affect the parents’ relationship with each other (21). Depressed mothers pay less attention to hygiene, including washing their hands before breastfeeding; thus, the germs can be transferred to the infant’s GIT during feeding and cause changes in the normal GIT flora (22). This alteration is considered a contributing factor for IC, and probiotic prescription is a relevant therapeutic strategy (23). In one study, paternal depression was associated with IC (24). The father can influence the development of IC indirectly by interacting with the mother during pregnancy; as well as directly after childbirth by lowering the father-infant interaction and not paying attention to its needs (25).
Another study also found that maternal self-efficacy in breastfeeding is associated with the incidence of IC. This is because of high maternal anxiety and incorrect breastfeeding technique (swallowing large amounts of air by the infant during breastfeeding and not taking the infant’s burp) (26, 27).
In this category, most studies have concentrated on the effect of migraine and smoking (28, 29, 30, 31). An infant who is born to a mother with migraine is hereditarily more prone to IC. The brains of such infants are more sensitive to environmental stimuli and manifest this sensitivity in the form of excessive crying (32). IC is recognized to be associated with the development of migraine in older ages (6). 
Moreover, maternal smoking presents various effects on the infant. Tobacco metabolites transmit through the placenta during pregnancy and post-birth by exclusive breastfeeding or secondhand smoke from the environment. As a result, motilin hormone levels are increased in the infant’s GIT; subsequently, it increases its motility and leads to abdominal pain (33, 34). Maternal anxiety affects fetal blood circulation during pregnancy and can lead to decreased infant neuronal development and disturb the mother-infant interaction (35). In this category, other factors, such as young maternal age (17, 18, 29), the experience of the first child (35), and failure to breastfeed (26) were classified. These make the mother incapable of fully managing the infant. For example, if the correct technique of breastfeeding is observed, it can protect the infant against colic (20, 36). In this regard, Kheirkhah et al. suggested that formula feeding increased the risk of IC, compared to exclusive breastfeeding (37). 
High Body Mass Index (BMI) (over 25 kg/m2) in pre-pregnancy (28) and vitamin B12 deficiency (13) are also risk factors for IC. Mothers with high BMI are less inclined to interact with their infants (38). They also have less control over their diet status during the breastfeeding period. These conditions can accelerate the IC incidence. Moreover, maternal vitamin deficiency is correlated with IC. Vitamin B12 causes the myelination of neurons; infants born to mothers with vitamin B12 deficiency may encounter a delay in neural system development (38). Delayed neural development is among the IC development causes.
The type of delivery, pregnancy, labor complications and the history of parity were classified in this category (17, 18, 39). Previous studies provided conflicting results concerning the type of delivery; therefore, further research is required in this respect (39, 40). In a study by Heydarian et al., infants with colic were mostly born through Normal Vaginal Delivery (NVD); however, it has been cited as a supportive factor via providing the normal intestinal flora while the infant passes through the birth canal (18, 41).
Moreover, high-risk pregnancies (eclampsia & pre-eclampsia) and delivery (vacuum or preterm delivery) have complications for the mother and infant after delivery. As a result, the mother-infant emotional relationship is affected and the risk of IC incidence is increased. Besides, increased mother parity reduces the risk of IC due to further experiences of coping with anxiety, stress management, and breastfeeding success (17, 42).
The paternal support of the mother is undoubtedly a significant factor that can reduce family anxiety and promote interaction with the infant (17). High social support of the mother is also considered as a supportive factor against IC due to benefiting from the experiences of other mothers (12). Additionally, maternal occupational and educational status fell into this category. Pregnant women who hold academic degrees, are studying in a school, or are involved in intensive and stressful jobs have a higher chance of bearing an infant with colic due to higher anxiety (17, 29). 
In one study, parental high socioeconomic status was a contributing factor to IC (35). They indicated more sensitivity toward their infants and more frequently visited physicians’ offices. Furthermore, these parents spend extensive time outside the home, which affects their interaction with each other as well as with the infant. 
This category is related to “maternal diet” during breastfeeding, i.e. investigated in two studies (11, 43). In one study, cow milk was found to be effective in the incidence of IC. The protein in cow milk can cause allergy and inflammation in the GIT (44). In some studies, a hypoallergenic diet and the elimination of cow milk were proposed as appropriate options for mothers whose infants experience the signs of atopy (45). 
Another study reported that protein intake presented a protective role on IC (46); therefore, further research in this field is warranted. The consumption of vegetables and fruits, such as celery, onions, and bananas (they contain derivatives that can be secreted into breast milk) causes abdominal cramps and increases the risk of IC. Bananas contain indigestible starch; most of which is not hydrolyzed in the GIT. Therefore, it causes bloating in the colon through gas production and eventually leads to abdominal pain (34, 47). In addition, chocolate consumption in one study was also associated with IC, which may be due to the presence of caffeine in its composition (30). The relationship between maternal diet and IC provided contradictory results in different studies; accordingly, further research is needed in this field. Overall, it was recommended to not manipulate the mother’s diet to reduce IC (48).
In the last category, the results of studies mostly referred to the maternal use of antibiotics during pregnancy and postpartum (20, 49). Taking antibiotics, changes the normal flora of the infant’s GIT, i.e. nourished by breast milk. Thus, the disorder in normal microbial flora can cause IC (50). Moreover, maternal iron supplement uptake during pregnancy was associated with IC in one study; however, that was not reported in other studies and requires further investigations (28).
This study had some limitations, including the lack of accurate definition of IC and medium quality in some studies. Furthermore, observational studies were assessed in this research and other types were disregarded. 

5. Conclusion
The present review results suggested that several parental factors were related to IC incidence. The most important factors predisposing to IC were parental depression, anxiety, smoking, maternal migraine, young age, primiparity, and low family support. A majority of studies focused on the effect of maternal factors on IC incidence, and paternal role has been less described. These characteristics were classified into 6 categories; these factors should be considered in future studies to implement appropriate prevention programs. However, several factors present conflicting effects on IC and require properly designed studies in the future.



Ethical Considerations
Compliance with ethical guidelines
This study was approved by the Research Ethics Committee of Esfarayen, Faculty of Medical Sciences (Code: IR.ESFARAYENUMS.REC.1398.012). All ethical principles were considered in this article.

Funding
This research did not receive any grant from funding agencies in the public, commercial, or non-profit sectors.
 
Authors' contributions
Conceptualization, supervision, and methodology: Mahbobeh Firooz, Seyed Javad Hosseini; Investigation, writing -review & editing, writing - original draft: All authors.

Conflicts of interest
The authors declared no conflicts of interest.


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