Recently, in Japan, the percentage of leanness has risen in young women, and the average birth weight has decreased. An increase in the risk of low birth weight has been reported in lean expectant mothers. In this study, we aimed to clarify the relationship between mother's physique at the beginning of pregnancy and the infant's physique, by focusing on sex differences. The participants were 3,722 mothers who attended health checkups for 18-month-old infants in an urban Japanese city. The participants were limited to those with full-term births, thereby excluding the influence of gestational length. A total of 1,287 mothers, with 621 boys and 666 girls, were analyzed. Public health professionals interviewed the mothers, and transferred the required information from their maternity passbooks. We examined the physical characteristics of the mothers and their infants. Partial correlation analysis, adjusted by gestational length and the mother's age at delivery, was applied to study the association between the mother's BMI and the infant's physique at birth. In the primipara group, only the boys showed significant positive correlation between the mother's BMI and the birth weight (P = 0.025) and the Kaup index (P = 0.035). In the pluripara group, only the boys showed significant positive correlation between the mother's BMI and the head circumference (P = 0.035). Thus, mother's physique may have a stronger influence on the physique of male infants, compared to female infants. The growth-promoting effect of the mother's physique is more apparent in the infants born to the pluripara.

In Japan, the percentage of leanness has been increasing in young women, and the percentage of low birth weight infants (< 2,500 g) has increased. Moreover, the average age of primiparas rose 3.5 years during the last 30 years. The purpose of this study was to clarify the relationship between maternal age and the influence of maternal pre-pregnancy physique on the neonatal physique of infants. Questionnaires were issued to the participants and collected when they submitted their gestational notifications at their local ward office in Kyoto Prefecture. After delivery, we obtained information on the course of the pregnancy and the neonatal physique of the infants from the participant's maternal passbooks. A total of 454 mothers (age 20 ≥) were analyzed: 161 young mothers (aged 20 to 29 years), 185 mothers (aged 30 to 34 years), and 108 older mothers (age ≥ 35). Overall, the mean rate of leanness (pre-pregnancy BMI < 18.5) was 23.8%. We found that birth weight was significantly lower in female infants, born to lean young mothers, compared to non-lean young mothers, whereas no significant difference was detected in other mothers (age ≥ 30), irrespective of pre-pregnancy BMI. By contrast, male infants, born to older lean mothers (age ≥ 35), showed significantly lower birth weight. Thus, maternal pre-pregnancy BMI exerts differential effects on the fetal growth (neonatal physique), depending on the maternal age and the sex of infants. We need to improve BMI in pre-pregnancy women, especially those in the twenties and 35 years old or over.

We have previously reported a positive correlation between the body mass index (BMI) of mothers and the Kaup index of infants at birth and that this correlation may have a stronger influence on the male infants, born to older mothers. Therefore, in this study, we aimed to clarify the correlation between maternal age and gender of the infants as maternal BMI and the Kaup index of infants from birth till 18 mo of age. This study was conducted from April 2010 to September 2011 in Japan. Public health nurses and registered dietitians interviewed the mothers individually under anonymous conditions, and they transferred the required information from the maternity passbook at the 18-mo health checkup. In male infants, significant positive correlations were demonstrated between maternal BMI at the beginning of pregnancy and the Kaup index of infants in mothers older than 35 y when the infants were at birth, at 4 mo and at 18 mo old. In female infants, there was no correlation between maternal BMI and the Kaup index of infants at birth. However, significant positive correlations were demonstrated between maternal BMI and the Kaup index of infants in mothers in their 20s and older than 35 y old when the infants were 4 and 18 mo old. Women who plan on pregnancy and medical professionals need to know that the maternal physique at the beginning of pregnancy affects the physique of the infants at birth and at 4 mo and 18 mo old by gender of the infants.