By Adam P. Matheny

Studies of premature twins’ development indicate that to be a premature twin may not be comparable to being a premature singleton. As a consequence, while parents and professionals must be mindful of how the stature of each co-twin compares with that of two singletons, they must further consider twins’ growth in infancy by making comparisons among twins themselves.

Within each pair of co-twins, it is not usual for one to be relatively heavier than his sister or brother, though within pair differences in weight are usually less than one kilogram. Many parents of twins in this situation may wonder if their smaller co-twin will ever equal his twin sibling in height and weight.

A look at the research shows that, at birth, identical twins do not appear to be more similar for length and weight than same-sex fraternal twins. During the following twelve months, however, identical twins become more similar and same-sex fraternal twins become somewhat less similar. In essence, twin pairs advance on a growth curve that is regulated genetically. Identical twins, by being genetically alike, tend to conform more closely to the same growth curve; same-sex fraternal twins, who are genetically less alike, conform less to the same growth curve. In fact, among some same sex fraternal pairs, it is not a surprise to find that one twin is larger one year and his co-twin is larger the next.

The most striking effect of the genetic influence on growth is seen by the rapid advance of a smaller twin in catching up with his larger co-twin. Ronald Wilson Ph.D., the late director of the Louisville Twin Study documented the ‘catch-up effect’ by tracing the growth of identical twins pairs in which one twin’s stature was relatively stunted compared to his sister or brother at birth. As measured at four successive ages during infancy, the smaller twin’s growth proceeded at a sharply accelerated pace so that his growth curve increasingly converged on that of his co-twin’s.

The progressive recovery from the disadvantages of smaller stature appeared to have been guided by the intrinsic genetic similarity of the identical twins. Although the smaller identical twin did not grow to become equal in height and weight to the larger co-twin in all cases, he did so more often than not.

Thus, within identical twin pairs, parents and professionals can expect that the smaller infant twin will tend to ‘catch-up’ in size with his larger twin sibling. At the same time, both he and his co-twin are offsetting the deficits of their being smaller at birth than most singletons.
For same-sex fraternal twins, it is also evident that both twins will make marked gains in infant stature. But the eventual stature of the smaller twin compared with the larger child is less predictable for them than it is for identical twins.

Although all of the influences on twin growth are not known, it is evident that parents could ideally best appreciate the growth of their own twins by comparing them with the growth standards of same-sex twins – at least up to the age of about eight years. Because growth standards for twins are not widely distributed (outside the Louisville Twin Study), many parents of infant twins will have to compare the growth of their own twins with the more commonly used standards widely available for single-born infants.

Although such comparisons are informative for tracing the ‘catch-up effect’ among infant twins, parents must remember that their twins’ long-term gains in stature are gradual, extend well beyond infancy and become apparent for height sooner than for weight.

The heavy news about birth weight

1. A few studies of twins measured from birth through childhood indicate that a twin’s average weight deficit at birth is about one kilogram.
2. This deficit is offset sharply during early growth so that after the first three months of life, the average deficit has been reduced to about half a kilogram.
3. The early period of rapid weight gain (during the first three months of life) presumably represents the twin’s escape from the suppressive environment of a twin pregnancy, access to more nourishment and an opportunity to grow in greater accord with the genetic growth potential provided by the parents. Whatever the reasons, this early infant period is the single time during which twins seem to gain weight faster than singletons.
4. A typical twin puts on more weight during the first year than during the entire next three years combined. Despite these gains, however, the twin’s weight deficit continues up to middle childhood, at which time the deficit is essentially erased.

The long and short of birth length

1. The impoverished prenatal environment of twins is less of an influence on birth length than it is on birth weight. On average, a twin at birth is about three centimetres shorter than a single-born infant.
2. The growth trend for twins’ length over the ages shows that twins make larger gains in length than singletons throughout the first two years. Thereafter, the catch-up becomes less pronounced.
3. Twins march steadily toward growing at a rate equal to singletons; by four years of age, most twins have effectively recovered from the deficit readily apparent at birth.
4. The determinants of an infant’s birth length and growth in length are less affected by a premature birth than are birth weight and growth in weight. Instead, his birth length and growth in length seem primarily determined by the child’s genetic endowment from his parents. Whether or not infants have a deficit in length, or whether or not they will ultimately be taller or shorter, may be predicted more accurately by considering parental height than any other factor. Unfortunately, the growth charts used for assessing a child’s development of length (or height) are not adjusted for parental statures.

Male-female twins
The patterns of growth differences within the pairs of opposite-sex twins present a different picture from that of same-sex twin pairs:

1. Sharp contrasts of infant growth occur within male-female pairs which are generally not equivalent to same-sex twins. For example, twin boys tend to weigh slightly more than twin girls at birth, even though both groups are equal in length.
2. During infancy, the boys’ gains in weight and length are appreciably accelerated over that of the girls.
   By six months of age, girl twins weigh, on average, about six hundred grams less than boy twins and are about three centimetres shorter. This size differential is maintained up to two years, after which the girl twins grow more rapidly than the boys.
3. Despite their later growth spurt, however, the girls typically do not reach a size equal to their male co-twins throughout childhood. The above differences in growth of boys and girls create differences of stature within opposite-sex pairs; such differences are noteworthy only when the size difference markedly favours the girl twin during infant growth.

 

Adam Mathenys has a Ph.D in psychology and is a professor of paediatrics at the University of Louisville of Medicine. He is also the director of the Louisville Twin study, one of the most comprehensive studies on twins ever conducted.