Seasonal variations of neuromotor development by 14 months of age: Hamamatsu Birth Cohort for mothers and children (HBC Study).

The present study aimed at investigating whether neuromotor development, from birth to 14 months of age, shows seasonal, cyclic patterns in association with months of birth. Study participants were 742 infants enrolled in the Hamamatsu Birth Cohort (HBC) Study and followed-up from birth to the 14th month of age. Gross motor skills were assessed at the ages of 6, 10, and 14 months, using Mullen Scales of Early Learning. The score at each assessment was regressed onto a trigonometric function of months of birth, with an adjustment for potential confounders. Gross motor scores at the 6th and 10th months showed significant 1-year-cycle variations, peaking among March- and April-born infants, and among February-born infants, respectively. Changes in gross motor scores between the 10th and 14th months also showed a cyclic variation, peaking among July- and August-born infants. Due to this complementary effect, gross motor scores at the 14th month did not show seasonality. Neuromotor development showed cyclic seasonality during the first year of life. The effects brought about by month of birth disappeared around 1 year of age, and warmer months seemed to accelerate the neuromotor development

Ambient temperature, precipitation, and duration of sunlight in Hamamatsu, Japan (34^o42.5′N 137^o43.1′E: 1981–2010). [31].

<p>Ambient temperature, precipitation, and duration of sunlight in Hamamatsu, Japan (34^o42.5′N 137^o43.1′E: 1981–2010). <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0052057#pone.0052057-Japan1" target="_blank">[31]</a>.</p

Gross motor scores at the 6th (circle), 10th (square), and 14th (X) months of life, as assessed by the Mullen Scales of Early Learning, predicted by trigonometric function of months of birth.

<p>Footnotes: Each curve and dot indicates predicted values of gross motor scores at the 6th, 10th, and 14th month regressed onto a trigonometric function of months of birth, adjusted for maternal age at birth, gestational age at birth and birthweight, with 95% confidence intervals. Statistics for trigonometric function, amplitude and phase in month at the 6th month assessment: F(2, 736) = 21.71, p<0.001, Amplitude = 0.40 (95%CI: 0.39 to 0.41) points, phase in month = 0.52 (95%CI: 0.50 to 0.54) months. At the 10th month assessment: F(2, 736) = 12.36, p<0.001, Amplitude = 0.50 (95%CI: 0.49 to 0.51), phase in month = –0.82 (95%CI: –0.85 to –0.79) months. At the 14th month assessment: F(2,736) = 1.21, p = 0.30, amplitude and phase were undetermined; predicted values are mean of gross motor scores adjusted for maternal age at birth, gestational age at birth and birthweight.</p

Mean gross motor scores at the 6th, 10th, and 14th months of age, and their statistical associations with potential confounders.

<p>Mean gross motor scores at the 6th, 10th, and 14th months of age, and their statistical associations with potential confounders.</p

Difference in gross motor scores between (a) 6 and 10 months of age and between (b) 10 and 14 months of age.

<p>Each curve and dot indicates predicted values of difference in gross motor scores between (a) 6 and 10 months or (b) 10 and 14 months of age regressed onto a trigonometric function of months of birth, adjusted for maternal age at birth, gestational age at birth and birthweight, with 95% confidence intervals. Footnotes. (a) Statistics for trigonometric function, amplitude and phase in month in difference between 6 and 10 months: F(2, 736) = 5.27, p = 0.005, Amplitude = 0.32 (95%CI: 0.31 to 0.33) points, phase in month = –2.57 (95%CI: –2.62 to –2.52) months. (b) Statistics for trigonometric function, amplitude and phase in month in difference between 10 and 14 months: F(2, 736) = 14.4, p<0.001, Amplitude = 0.60 (95%CI: 0.57 to 0.63) points, phase in month = 4.49 (95%CI: 4.45 to 4.54) months.</p

Characteristics of the participating infants [N = 742].

*<p>Cumulative number of children who have already been weaned from breastfeeding until each month of age.</p

Model statistics (R-squared, F-test and p-value), test statistics for an addition of trigonometric functions of months of birth (H₀: A₁ = A₂ = 0), coefficients for the trigonometric functions, the amplitude and phase of trigonometric function of gross motor scores at 6, 10, and 14 months of age, and for differences between 6 and 10 months, and between 10 and 14 months of age.

<p>Footnotes for <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0052057#pone-0052057-t003" target="_blank">Table 3:</a> Each model was adjusted for maternal age at birth, birthweight and gestational age at birth.</p