Discussion
By keeping the delivery room temperature at around 26–30°C, the rooming-in temperature at 24°C and nursing the infant skin to skin or dressed and covered in a cot, the median and mean rectal temperatures were close to 37.0°C during the first 24 hours of life in these healthy infants born at term. The 2.5th–97.5th range was close to what has been defined as the normal range of 36.5–37.5°C by WHO, but 28% of the infants experienced hypothermia according to the WHO definition. Hypothermia occurred mainly during the first 8 hours of life and was usually mild. Low birth weight was the major risk factor for hypothermia, while sleeping, male sex or being born at night had a minor impact. Skin-to-skin care had a protective effect, while mode of delivery had no effect on rectal temperature. Hyperthermia occurred in 12% of the infants, usually after 8 hours, and was mainly associated with high birth weights.
The mean rectal temperature was similar to mean axillary and rectal measurements in similar single centre studies,11–13 21 22 but no previous attempts have been made to scientifically define a normal range12 13 or to construct a percentile chart for body temperature for a given thermal environment. Instead, it has been common practice to calculate the percentage of infants with hypothermia or hyperthermia at various points in time and compare results with various normal body temperature standards given in textbooks. Such normal standards were summarised by Li et al who concluded that they were not evidence based.13
The rectal temperature during the first 24 hours of life in our study was very similar to that of a UK study at 2 and 24 hours of age,11 and that of 27 Swedish term-born infants monitored for 48 hours after birth,22 but higher than temperatures at 2 and 24 hours after birth in an intervention study to combat hypothermia in Nepal.23 In the Nepalese study, the infants received immediate drying under radiant heat, swaddling including the head and nursing on the mother’s chest, but 22% of the infants weighed <2500 g, and the mean room temperature was only 17.8°C. In the UK study,11 routines included swaddling, overhead heaters and incubators, and the mean ambient temperature was ~2°C higher than in our study. Their incidence of hypothermia was lower than in our study, but only 22% of temperature recordings were rectal. In the Swedish study, the ambient temperature was ~1°C lower than in our study.22 Other relevant studies used axillary temperatures or methods of thermal protection which were not comparable to our study9 12 13 or did not report ambient temperatures.1 9 13 24 25
Our finding that a quarter of the infants had temperatures below 36.5°C during the first 8 hours demonstrated the difficulty of maintaining what is considered a normal body temperature immediately after birth. This finding is in line with research which has demonstrated that infants are unable to fully compensate for heat loss by increasing their metabolism during the first hours after birth, while a gradually increasing basal metabolic rate contributes to the maintenance of a normal body temperature from 8 to 12 hours of age.26 However, the variation between infants is large, especially during the first day of life,27 as it was in our study.
The clinical consequences of mild hypothermia are difficult to ascertain, but mild hypothermia has been associated with hypoglycaemia4 and hyperbilirubinaemia.6 However, the high incidence of low temperatures despite environmental control near the recommendations of WHO underscores that the risk of hypothermia may be high under less favourable conditions. Further studies are needed, preferably multicentre studies to be able to evaluate local variations in thermoregulatory practices.
We measured rectal temperature because it is traditionally considered the gold standard for assessing the body temperature.28 Axillary temperature is commonly recommended because of ease and safety if accurate measurement is not essential.5 Axillary temperature is generally lower than rectal,29 30 and our percentile values had probably been lower and the rates of temperatures in the hypothermic range higher if we had used axillary instead of rectal temperature. However, there may be no simple correlation between these measurements because they represent different body compartments.31 Rectal measurement is considered safe if the thermometer is not inserted further than 2–3 cm into the rectum,32 and WHO recommends 2 cm insertion.5
The strengths of this study included the large number of participants and the consistent way of obtaining measurements. The study would have gained from simultaneous measurements of rectal and axillary temperatures since axillary measurement is a common screening procedure, but rectal measurements may be preferred when accuracy is essential.5 33 WHO recommends a rooming-in temperature of at least 25°C.5 However, such temperatures are often difficult to maintain because they feel uncomfortable for adults,5 34 and we suggest that 24°C, as used in our study, is sufficient if other recommended ways of thermoprotection are adhered to.