Many studies have found a relationship between birth order and birth weight, with the later-born children in a family tending to be larger, and birth weight has some correlation with eventual height.
For example, a 1988 study at the Children’s Hospital of the University of Kiel, Germany, investigated adult height in families with three or more adult children.
The researchers found that the first-born siblings had a statistically significant deficit in average height as compared with the average of all the siblings, but the gap amounted to only a fraction of an inch.
That study also found that men tended to increase in height with increasing birth order, while women did not, beyond the second born. The researchers concluded that birth order is a factor that contributes significantly to the variance of adult height among siblings.
The reasons for the tendency of younger siblings to be born larger are not clear. In an Israeli study that adjusted for a number of factors —like maternal age, education, marital status, religion, smoking, height and prepregnant weight, gestational age, and sex—birth weight increased with increasing birth order both in a large cross section of newborns and in a group of large families, where many of the possible variables would presumably be constant.
