Early Human Development 60 (2000) 35–42www.elsevier.com/ locate /earlhumdev

The effects of pre- and post-natal sunlightexposure on human growth: evidence from the

Southern Hemisphere

a , a b a*Karen E. Waldie , Richie Poulton , Ian J. Kirk , Phil A. SilvaaDunedin Multidisciplinary Health and Development Research Unit, Department of Preventive and

Social Medicine, Dunedin School of Medicine, University of Otago, Dunedin, New ZealandbDepartment of Psychology, University of Auckland, Auckland, New Zealand

Received 14 February 2000; received in revised form 19 July 2000; accepted 20 July 2000

Abstract

Several recent studies have reported a causal association between stature and month of birth.Perinatal exposure to sunlight has been suggested as the principal factor underlying thisrelationship, although the mechanisms involved remain a matter of debate. The longitudinaldesign of the present study allowed us to directly test the influence of perinatal sunlightexposure (and other meteorological and behavioural factors) on body size at birth and at regularintervals up to age 26. The findings confirmed that pre-natal sunlight is one of the mostsignificant determinants of height. However, the trimester of greatest influence differsdepending on the age at which study members were measured. 2000 Elsevier ScienceIreland Ltd. All rights reserved.

Keywords: Sunlight; Month-of-birth effect; Development; Cohort; Pre-natal; Body size

1. Introduction

The intriguing finding that body size varies according to season of birth in bothhumans [1,2] and animals [3] has led to speculation concerning the role of perinatal

*Corresponding author.E-mail address: kwaldie@gandalf.otago.ac.nz (K.E. Waldie).

0378-3782/00/$ – see front matter 2000 Elsevier Science Ireland Ltd. All rights reserved.PI I : S0378-3782( 00 )00102-X

36 K.E. Waldie et al. / Early Human Development 60 (2000) 35 –42

exposure to sunlight for development. The seasonal variation has been hypothesisedto result from environmental exposure (e.g., sunlight [1], total energy reaching earthor fluctuations in temperature [3]) influencing hormonal expression between motherand developing foetus.

Some studies, however, have failed to find any seasonal effect on body size [4,5].This issue is complicated further by incongruities both between and within studies.For instance, Weber et al. [1] showed that height varied up to 6-mm depending onwhether 18-year-old Austrian males were born in spring (i.e., maximum height inApril) or autumn (minimum height in October). Although their results indicated thatpeak sunlight duration occurred in July (thereby corresponding to the first month ofgestation and the fourth post-natal month in the tallest males), the authors speculatedthat the effect was due to ‘‘influences extending from the time of late pregnancy tothe first post-natal year’’. A further report [2] from the Northern Hemisphere foundthat the stature of a sample of infants was greatest both in the spring and in theautumn.

The initial aim of the present study was to determine if stature fluctuates withseason of birth in a Southern Hemisphere population to the degree that has beenreported in the above mentioned Northern Hemisphere studies. Further, by cross-correlating fluctuations in daily sunlight hours during pregnancy with the height andweight of infants at birth, the period of maximal influence of sunlight during gestationcan be gauged.

Thus, in study 1 we sought to determine the solely pre-natal effects of sunlight onthe birth size of a cohort of all infants born between 1967 and 1978 in Dunedin, NewZealand (a city with a population of approximately 120 000). To directly test thisrelationship, average hours of bright sunshine recorded for each month during thesame period were obtained from the New Zealand meteorological service [6].Variation in average monthly sunshine hours between consecutive years in Dunedin(e.g., April 1971 5 169 h versus April 1972 5 82 h) illustrates the importance ofgathering data in this way. Typically the maximum duration of sunshine in Dunedin(latitude 5 45 54 South, longitude 5 170 31 East, 2 m above MSL) is in January(summer) and the minimum is during June (winter).

In study 2, data from a subset of this population (who were subsequently enrolledin the Dunedin Multidisciplinary Health and Development Study [DMHDS] [7]) wereemployed. The aim of study 2 was to investigate the influence of season of birth (andother meteorological and behaviour factors) on height and weight during development(up to age 26 years). Of particular importance was the longitudinal design of thisstudy in that it allowed us to elucidate which period of sunshine duration (pre- versuspost-natal) exerts the most influence on body size.

2. Method

2.1. Study 1

Infants born at the major maternity hospital in Dunedin between 1st August 1967and 31st July 1978 were included in this study (n 5 20 021). Data for births in 1974

K.E. Waldie et al. / Early Human Development 60 (2000) 35 –42 37

were not available. Infants were examined with regard to anthropomorphic features(e.g., length, weight, gestational age, head circumference), and their primarycaregivers were interviewed within 24–48 h of delivery.

Cross-correlation functions (CCF’s) [8] were calculated from the Fourier trans-forms of the mean monthly height, weight and sunlight data between August 1967and June 1978. CCF’s were constructed such that sunlight data was referenced toheight and weight data. Thus, the lag of maximum mean monthly birth height, orweight, relative to maximum mean monthly sunlight hours is indicated by the time atwhich the first correlation maximum at positive shift occurs.

2.2. Study 2

A sub-sample consisted of members of the DMHDS, a longitudinal investigation ofthe health, development, and behaviour of all individuals born between 1st April 1972and 31st March 1973. This cohort has been assessed individually on a wide variety ofpsychological, sociological, and medical measures at 2-year intervals from age 3(n 5 1037) to age 15 (n 5 976), and subsequently at 18 (n 5 993), 21 (n 5 992), andmost recently at age 26 (n 5 980). Follow rates have been high with, for example,96.2% of the living cohort seen at the most recent assessment.

In order to test for linear relationships between mean pre-natal sunshine hours andstature throughout development (birth to age 26), Pearson Product Moment Correla-tion coefficients were calculated for height and (1) mean sunshine hours at eachpre-natal month, and (2) mean sunshine hours for each month 1-year postnatally.

Height data at each assessment age were also subjected to separate multipleregression analyses. In addition to mean sunlight hours, we included two otherpotential meteorological predictors of body size: average incoming atmosphericradiation (from sun and sky received on a horizontal surface in Langleys per day) andmean monthly air temperature (taken at 9:00 am in Dunedin). For each of the threevariables, the mean monthly data were averaged to correspond to the first, second andthird pre-natal and first post-natal trimester periods for each Study member.

We first ran a backward selection multiple regression on the meteorological data.The variables that remained in the final model were then subjected to further multipleregression analyses that included two potential behavioural predictors of body size:(1) number of weeks of breast-feeding; and (2) number of cigarettes smoked by themother during pregnancy. Factors such as maternal age, gestation age, ordinalposition in the family, and total number of children were not included in the modelbecause earlier studies with this cohort [9,10] found little relation between thesevariables and physical growth.

2.2.1. Feeding methodEarly infant feeding records were obtained by community health care nurses and

re-checked via maternal report at the 3-year assessment. A total of 53% of mothersbreast-fed their infant.

2.2.2. Smoking behaviourAt the age 9 assessment (1981–1982), mothers (n 5 765, 80.3%) were asked to

38 K.E. Waldie et al. / Early Human Development 60 (2000) 35 –42

recall the average number of cigarettes smoked per day during each trimester ofpregnancy and during the first three post-natal months. Results showed that 21.3% ofthe mothers reported smoking (more than ten cigarettes per day) in the first trimesterof pregnancy, approximately 20% reported smoking in the second and third trimester,and 23.9% reported smoking in the first 3 months post-birth.

3. Results

3.1. Study 1

Spectral analyses [8] revealed that monthly means for neonate height and weightvaried sinusoidally at the same frequency as that of the monthly variation in meanbright sunlight hours.

Peak height and weight lagged the month of peak sunlight hours by between 6 and9 months (Fig. 1). This indicates that any facilitory effect of sunlight hours on birthheight and weight was most evident when maximal sunlight was coincident with theearly months of pregnancy (i.e. the first trimester).

Fig. 1. Mean monthly birth height (A), sunlight (B) and birth weight (C) and the cross-correlationfunctions (CCF) between mean sunlight hours and mean height (D) and weight (E) at birth betweenAugust, 1967 and June, 1978 (data for 1974 were unavailable). The lag of maximum mean monthly birthheight, or weight, relative to maximum mean monthly sunlight hours is indicated by the time at which thefirst correlation maximum at positive shifts occurs.

K.E. Waldie et al. / Early Human Development 60 (2000) 35 –42 39

3.2. Study 2

To determine if this relation remained during growth, we further investigated thesub-sample of children enrolled in the DMHDS. As would be expected from thespectral analysis of the full sample, greater exposure to sunshine during earlypregnancy was also associated with increased length at birth (Fig. 2). That is, thecorrelation between mean sunshine hours (during each month of gestation) and lengthat birth was positive during the first three pre-natal months and negative during thelast six pre-natal months. Consistent with an earlier report [1], this trend was alsoapparent when Study members were assessed at age 18.

In contrast, Fig. 2 shows that at each assessment period throughout childhood andearly adolescence (i.e., age 3 until age 15), the correlation between mean sunshineand stature was negative until about the sixth month of gestation and then positivethereafter. Height measured at age 11 shows this effect most strongly. Fig. 3 clearlydemonstrates that the average z-scores of height and weight at age 11 linearlyincreased as the duration of last trimester sunlight hours increased [F(11, 653) 5

2.68, P 5 0.002].Results from multiple regression confirmed that the strongest determinant of stature

throughout early growth and puberty was average duration of third trimester sunshinehours (r 5 0.117, b 5 0.09, P , 0.01). The other significant predictors of childhoodheight that remained in the final regression model were duration of breast-feeding(r 5 0.065, b 5 0.06, P , 0.05) and average incoming atmospheric radiation duringthe 3rd trimester of gestation (r 5 0.109, b 5 0.03, P , 0.10). The correlationbetween sunlight and atmospheric radiation was strong (r 5 0.825, P , 0.001), yetboth factors contributed independently to the final model.

Factors such as mean monthly air temperature (r 5 0.079, b 5 0.02, n.s.), andnumber of cigarettes smoked by the mother during pregnancy (r 5 2 0.046, b 5 2

0.01, n.s.) were not included in the final model. Analyses did reveal, however, thatpre-natal maternal smoking was significantly associated with a significant decrease inbirth weight (P , 0.01), and reduced length at birth was associated with maternalsmoking during the final trimester of gestation (P , 0.05).

Interestingly, correlation data as well as separate multiple regression revealed nosignificant relation between height at any age and average sunshine hours during thefirst nine post-natal months. Also consistent with the findings by Weber et al. [1], thelargest positive influence that post-natal sunlight had on adult stature was the fourthpost-natal month (r 5 0.021, n.s.). Because maximum sunlight occurring during thefourth post-natal month was strongly associated with birth length (r 5 0.059, P 5

0.06) and showed a significant negative correlation with height at age 3 (r 5 2 0.072,P 5 0.029), we are reasonably confident that the month of birth effect on height isdue to pre-natal environmental exposure.

4. Discussion

Our findings revealed that the birth length and weight of this Dunedin cohort wasstrongly influenced by the amount of bright sunshine coincident with the early months

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Fig. 2. The correlation between height (z-scored) and the average duration of sunshine hours during each of the 9-months of foetal gestation (each bar represents onepre-natal month). Pearson Product Moment Correlations are shown for each of the ten assessment periods (birth to age 21). The duration of sunshine hours was recordedin Dunedin, NZ between 1971 and 1973.

K.E. Waldie et al. / Early Human Development 60 (2000) 35 –42 41

of pregnancy. In the sub-sample of members enrolled in the DMHDS height variedup to 8-mm depending on month of birth, with maximum length in October(conception during mid-summer) and minimum length in January (conception duringshorter autumn days). That we found significant seasonal variation in a relativelysmall sample size contradicts the assertion by Weber and associates [1] that the monthof birth effect can only be discerned with ‘‘very large sample sizes’’, and confirms thevalidity of the sunlight effect in a Southern Hemisphere sample.

The mechanism by which sunlight during first trimester gestation contributes toboth birth length and adult stature remains to be determined. However, we speculatethat sunlight exposure during the first trimester of gestation may stimulate growthhormone release by inhibiting production of maternal pineal melatonin [11–13]. Thispotential, laid down during the period of most rapid cell growth and differentiation,may re-emerge after puberty to determine final stature.

In contrast to the findings at birth and adulthood, we found that growth throughoutearly childhood and adolescence was maximal when peak sunshine duration tookplace during the last trimester of gestation. Importantly, last trimester sunshine hoursas well as average incoming atmospheric radiation accounted for a greater proportionof the variance in body size than either duration of post-natal breast-feeding, ornumber of cigarettes smoked by the mother during pregnancy. Growth hormoneproduction and release may therefore interact with atmospheric conditions at differentpre-natal periods. This, as well as the finding that radiant energy and sunlight wereindependent (though highly correlated) predictors of height, suggests that themechanism(s) underlying the Month-of-Birth effect may be more complicated thanpreviously realised.

In sum, it appears that the effect of the season of birth on stature is primarilymediated via pre-natal rather than post-natal meteorological conditions. This accordswith previous observations that differential exposure to light does not affect the

Fig. 3. The deviation of the average z-scores of height and weight at age 11 by the mean total sunlighthours in the final trimester of gestation. The birth length and weight of the cohort were significantlycorrelated with body size at age 11 (height: r 5 0.26, P , 0.001; weight: r 5 0.21, P , 0.001).

42 K.E. Waldie et al. / Early Human Development 60 (2000) 35 –42

growth of premature infants (measured 1-year post-conception) [13]. Perhaps moreintriguing was our finding that pre-natal duration of sunshine differentially influencedstature depending on the age at which participants were assessed. Further studies areneeded to pursue and elucidate these issues.

Acknowledgements

The Dunedin Multidisciplinary Health and Development Study (DMHDS) issupported by the Health Research Council of New Zealand. The authors thank thelate Dr. Patricia Buckfield for collecting the neonate data and the DMHDS Studymembers for their participation and continued support.

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