The phase of pregnancy is a vital event in people's lives, both for future mothers and also fathers. Therefore, the moment of birth as well as the caring of the baby define a period of crucial change for progenitors. In this sense, emotions reflect what happens to us; they are signs and symptoms which provide feedback concerning the way we confront different situations. The events lived during the perinatal stage (especially during the first hours after birth) bear great relevance in order to understand the process of changes experienced by progenitors of full-term babies. But what about the parents of premature infants? Prematurity increases the biological and psychological risk factors in babies and parents. The study of preterm birth will allow us to observe the psycho-emotional consequences (disorders of early interaction, exercise of parental roles, the ability to cope with stress, amongst others) that are generated by the biological risk factors (neurodevelopmental problems). This will make it possible to design procedures of intervention adapted to the needs of children and parents (Karen, Tyano, & Sirota, 2000).

It is estimated that nearly 15 million premature babies (that is to say, those who are born with less than 37 weeks of gestation) are born in the world each year (Platt, 2014). In 2015, WHO reported that the rate of preterm births fluctuated between 5% and 18% (WHO, 2015). In Europe, this rate varies between 5% and 11% depending on the country (Zeitlin, Mohangoo, & Delnor, 2010). According to the National Institute of Statistics (Instituto Nacional de Estadística, INE), each year during the last decade, more than 30,000 children were born prematurely in Spain and more than 1000 of them in the Canary Islands. This represented 6.5% and 6.8% of the population, respectively in 2014 (INE, 2014). There is broad consensus that, at birth, age is more important than the weight, because the former is what really determines the maturity of the major organs. The immature child has a greater risk of not successfully completing the process of adaptation to external and autonomous life, so this critical phase can have medical and psychological sequelae for the rest of the baby's life (Horbar et al., 2002).

The emotional situation of preterm babies’ parents is subject to high levels of uncertainty and ambiguity. In a couple of hours, the preterm baby's situation can change, whether with a positive or tragic end, or it can continue in an imprecise and uncertain way. How do parents react to this experience of uncertainty, stress and fear? Is it advisable to proceed with a professional psychological plan? How should this plan deal with this situation? Currently, there is little empirical evidence that helps to answer these questions.

Psychological stress is considered as “a particular relationship between the person and environment that is appraised by the person as taxing or exceeding his or her resources and endangering his or her well being” (Lazarus & Folkman, 1984). When a person considers that he/she does not have enough resources to face the situation, his/her well-being can be negatively affected. Also, if this psychological tension is not dispelled appropriately, it can lead to mental diseases related to anxiety and depression (Boullosa, 2004). Besides, it is likely that the birth of a baby (especially when it is a preterm birth) increases the stress levels and the negative emotional status of the parents, thus making the development of a post-traumatic stress disorder possible (Pierrehumbert, Nicole, Muller-Nix, Forcada-Guex, & Ansermet, 2003). Probably, the adequate management of these stressful events can help to create a more adapted and healthy management of the situation.

Finally, Plutchik's evolutionary theory of emotions contributes with a complementary conceptual frame to this quandary. In the case of premature births, it appears that interactive behaviors are worthy of particular attention, and their origin depends on multiple factors. The emotional suffering of parents, especially the mother's suffering, has been attributed to the immaturity of the premature infant, as well as the early separation of the child from the parents (Crnic, Ragozin, Greenberg, Robinson, & Basham, 1983; Miles & Holditch-Davis, 1997; Muller-Nix et al., 2004). There are studies comparing the behavior of preterm infants’ mothers with that of mothers with healthy children born at term. The former are presented as more active, more vigilant and less sensitive to answer inadequately to certain baby's necessities (take longer to perceive and respond to the baby's physical and emotional alterations) (Joo, Choi, Rha, Kwak, & Park, 2015; Muller-Nix et al., 2004; Wijnroks, 1999).

The main aim of this study is to compare the levels of stress and the emotional state between the parents of premature babies and parents of infants born at term. This comparison is made in two different instances; firstly during the first days of the baby's life (12–72h) and finally after 6 living weeks. This will allow us to recognize the evolution in both groups.

More specifically, what this paper is trying to detect is whether there is a higher rate of positive emotions instead of negative ones in the group of parents of full-term neonates, as compared to the preterm one, and whether these differences are still present with the passing of time. Also whether there are any existing differences between the stress perceived by the preterm group in opposition to the full-term group, both in the first and second phase (first hours against 6 weeks).