Umeå universitets logga

Objective: Human deliveries commonly occur during the dark period of the day. Melatonin hormone is released as a circadian signal from the human brain, and has been shown to have a synergistic effect together with oxytocin in facilitating the uterine contractions during labor. Melatonin is inhibited by light and energized by dark. During winter the numbers of light hours per day in Sweden are few, but during the summer they are numerous. In Tanzania there is no season difference.

Study Design: In 2011,452 Tanzanian and 919 Swedish healthy primiparas with a normal pregnancy and a spontaneous onset of labor were included in the study. Time and date together with the length and outcome of delivery were studied, and a statistical comparison was made.

Results: 47% of the Swedish deliveries started in early morning, and 48% ended at midnight. The median time of active labor was significantly longer during the summer season (May to August vs. November to February, p=0.05). The frequency of labor dystocia was increased during summer season, and significantly more in the region on Sweden with midnight sun (29 vs. 40%, p=0.03).

48% of Tanzania labors started at midnight, and 41% ended at lunch time. Among Tanzanian women, no difference in median time of active delivery (p=0.5) or the frequency of dystocic labors was shown, according to season of the year (13.7 vs. 14.1 %, p=0.8).

Oxytocin was used in 68% of the Swedish deliveries, and the use was higher during the summer season. Oxytocin was used in 31% of the Tanzanian deliveries. No difference in use of oxytocin was shown according to season.

Conclusion: In this project a variation of labor dystocia according to season of the year has been studied. A significant overrepresentation of prolonged and dystocic deliveries were shown during the summer season in Sweden. This new knowledge may be of a great importance, when handling a dysfunctional labor.