Cortisol regulation in 12-month-old human infants: Associations with the infants’ early history of breastfeeding and co-sleeping

R Beijers, JM Riksen-Walraven, C de Weerth (2013). Cortisol regulation in 12 month-old human infants: Associations with the infant’s early history of breastfeeding and co-sleeping. Stress: The International Journal on the Biology of Stress, 16:3, 267-277.

Hypothesis

More weeks of breastfeeding and co-sleeping would contribute positively to HPA-axis regulation later in life. More weeks of co-sleeping and breastfeeding would predict a less strong cortisol reaction and a quicker recovery.

Design: Cohort, prospective longitudinal

Variables Measured, Instruments Used

1. Breastfeeding: Daily diaries from birth to six
2. Co-sleeping: Daily diaries from birth to six months
3. Psychological stressor and Infant Attachment: Strange Situation Procedure (SSP)
4. Cortisol: Saliva collected four times, before and after SSP
5. Quality of Maternal Caregiving: Videotaped bathing sessions
6. Maternal Depression: Edinburgh Postnatal Depression Scale (EPDS) at three, six and 12 months postpartum           
7. Infant Attachment: Videotapes of the SSP                                             
8. Infant Night waking: Daily diaries from birth to six months and again at 51-52 weeks
9. Infant Temperament: Infant Behavior Questionnaire Revised (IBQ-R) completed at three, six,  and 12 months

Participants

• N=193
• Participant Ages: 12-month-old infants
• Location: Nijmegen and Arnhem, Netherlands and surrounding cities
• Eligibility: Pregnant mothers with uncomplicated, singleton pregnancy, no drug use and no current physical or mental health problems
• SES: Not available
• Additional: Approx. 97% of the mothers lived with a partner and 75% of mothers had a college education

Limitations

1. Low generalizability. The generalizability of the study is limited since almost all mothers were highly educated and lived together with their partner.
2. Maternal report. The daily diaries recording the infant’s breastfeeding, co-sleeping, and night waking data were based on maternal report.
3. Potentially confounding factors:  It is unclear from our study what exactly happens during the night waking. For example, some mothers may at some point have employed sleep training techniques involving systematically ignoring infant signals of distress and night waking, or controlling the frequency and timing of resettling.
4. Missing father’s information: The role of the fathers in the nightly sleeping and feeding practices remains unrevealed in our data collection.

Findings

1. To our knowledge, this study is the first to show that an early history of co-sleeping and breastfeeding during the first six months of life is associated with infant cortisol regulation at 12 months of age.
2. More weeks of co-sleeping predicted lower cortisol reactivity to the SSP, while more weeks of breastfeeding predicted quicker cortisol recovery.
3. In concordance with previous studies (Van Bakel and Riksen-Walraven 2004; Laurent et al. 2012), our study showed that the SSP induced a cortisol reaction in 12-month-old infants, with a significant increase in the salivary cortisol levels from prestressor to postressor samples, and a significant decrease thereafter.
4. Higher cortisol reactivity was predicted by having more siblings, less negative affectivity, and more night wakings during the first six months.
5. A trend was found for current co-sleeping at 12 months of age to be related to high cortisol reactivity, thus co-sleeping might have different consequences for HPA-axis development at different ages.
6. Insecure attachment, co-sleeping at 12 months of age, longer time since last sleep and higher prestressor cortisol showed a trend to be related to higher cortisol reactivity.