Influence of family size and birth order on menarcheal age of girls from Bilbao city (Biscay, Basque country).

From a sample of 895 girls from Bilbao 9.5 to 18.5 years of age, the possible influence of family size and birth order on menarcheal age was analyzed. The earliest mean ages of menarche occurred in girls from families of one or four or more children, or who occupied the first or last birth order in their families. Later mean ages of menarche occurred in girls belonging to sibships with two or three children or who occupied the second or third birth order in their families, the latest being girls belonging to sibships of three children or of third birth order. This may be due to the variations in the interval of time between births, leading to better care and less stress for the youngest individual of the family and, hence, may be indicative of psychological as well as biological factors affecting maturation. Am. J. Hum. Biol. 11:779-783, 1999. Copyright 1999 Wiley-Liss, Inc.