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Intrauterine growth rates and mortality
15 0
Letters to the editor
REFERENCES
1. Segal, S., Blair, A., and Roth, H.: The metabolism of galactose by patients with congenital galactosemia, Am. J. Med. 38: 62, 1965.
Intrauterine gro~vth rates and mortality
To the Editor: In their article Drs. B~a Van den Berg and J. Yerushalmyl presented a very clever and tricky method for dividing a neonatal population in such a way that the influence of gestational age on neonatal mortality could be examined independently of birth weight. They rightly claimed that the groups formed by their method--equalized for birth weight and differing widely in the distribution of their gestational age--represented groups of different intrauterine growth rates. Having started from this point and after a jump (in their reasoning), they ascribed all the observed differences in neonatal mortality and subsequent developnlent to the differences in intrauterine growth rates. Surely, the primary difference between their groups is gestational age. Thus their title, "The relationship of the rate of intrauterine growth of infants of low birth weight to mortality, morbitity, and congenital anomalies," and such statements as, "most striking is the extremely high risk of neonatal mortality for infants of fast intrauterine growth rate" are quite misleading. What they have actually shown is the extremely high risk of neonatal mortality for the more immature infants. Apart from this unwarranted substitution of growth rate for immaturity, another point requires comment. The authors chose to study the group of infants of birth weights between 1,600 to 2,500 grams. By cutting this piece from the natural population, they produced a completely artificial one. The effect is obvious if one plots the gestational age distribution of their 4 groups. How artificial this population is can best be demonstrated by the following example: The authors claimed that their group I (short gestation quartile) had a very fast intrauterine growth rate. This is true only in comparison to their other groups; in comparison to the natural popu-
The Journal of Pediatrics July 1967
2. Baker, L., Mellman, W. J., Tedesco, T. A., and Segal, S.: Galactosemia: Symptomatic and asymptomatic homozygotes in one Negro sibship, J. PEDIAT.68: 551, 1966.
lation, this was not at all a very fast-growing group. In the curves of intrauterine growth from Denver, 2 it lies near the seventy-fifth percentile. If we take for comparison the curves from New York City, a from which the authors derived their material, their group I (fast growing) lies at the fiftieth percentile, groups II and III are on the twenty-fifth and tenth percentile, respectively, and group IV is below the tenth percentile and corresponds therefore with the "intrauterine growth retardation" group. Hence, it seems that the conclusions of the authors are valid as far as the group with intrauterine growth retardation is concerned. In the other groups only the effects of gestational age were in fact observed. It seems that there is no method for separating the effects of growth rate from that of birth weight and gestational age. We have to compare either infants of the same birth weight but differing in gestational age or infants of the same gestational age but differing in birth weight. There is certainly great scope in studying the group of neonatal infants (apart from the wellknown cases born to diabetic or prediabetic mothers) with what appears to be a very fast intrauterine growth rate; this can be accomplished only by a prospective study with careful evaluation of each infant of this group with all the available criteria of maturity. No retrospective playing with numbers is going to solve the problem. Such a study is all the more important, as we cannot at present decide betwee~l the "rational" intrauterine growth curves of the Denver2 group and the unexpected curves--with wider distribution at early gestational ages--of New York, Baltimore, s or Newcastle? If such a group really exists, then these apparently unacceptable curves, with the almost flat ninetieth percentile, will be explained by the premature birth of most of the fast-growing fetuses, when ~hey approach the usual weight for a full-term infant. t am sure that Drs. Van den Berg and Yerushalmy will be able to provide this valuable information when they will analyze in more de-
Volume 71 Number 1
Letters to the editor
tail their data from the child health and development studies in Oakland. T. VALAES, M.D., D.C.H.
DIRECTOR~
"QUEENANNA
MARIA"
INSTITUTE OF CHILD HEALTH AGHIA SOPHIA CHILDREN'S HOSPITAL ATHENS, GREECE 6 0 9
REFERENCES 1. Van den Berg, B. J., and Yerushalmy, J.: The relationship of the rate of intrauterine growth of infants of low birth weight to mortality and congenital anomalies, J. PEDIAT. 69: 531, 1966. 2. Lubchenko, L. O., Honsman, C., Dressier, M., and Boyd, E.: Intrauterine growth as estimated from liveborn-birth-weight data at 24 to 42 weeks of gestation, Pediatrics 32: 793, 1963. 3. Battaglia, F. C., Frazier, T. M., and Hellegers, A. E.: Birth weight, gestational age, and pregnancy outcome, with special reference to highbirth-weight-low-gestational-age infant, Pediatrics 37: 417, 1966. 4. Neligan~ G.: A community study of the relationship between birth weight and gestational age, in Dawkins, M., and MacGregor, W. G., editors: Gestational age, size, and maturity, Clinic in Developmental Medicine, No. 19, 1965.
Reply To the Editor: We will not comment on such statements as "clever and tricky," and "played with numbers." When Dr. Valaes says, "Surely, the primary difference between their groups is gestational age," he forgot to add the crucial words "to attain their identical birth weight." Our main purpose was to investigate the interaction of birth weight and gestational age as they relate to mortality, morbidity, and congenital anomalies. In order to do that, we had the choice, in the words of Dr. Valaes, "to compare either infants of the same birth weight but differing in gestational age or infants of the same gestational age but differing in birth weight." We elected to use the first of these two methods because the relationship of birth weight to outcome far outweighs that of gestational age. We therefore eliminated the birth-weight factor by equalizing the groups for birth weight in order to bring into sharper focus the interaction with gestational age. The result, as we noted in the
15 1
paper, is that "It took the fetuses of the fourth gestational quartile 6 to 8 weeks longer than those of the first gestational quartile to attain the same birth weight. The 4 groups, therefore, differ primarily in their rate o[ intrauterine growth." Apparently Dr. Valses is in agreement with this view for he says, "They rightly claimed that the groups formed by their method--equalized for birth weight and differing widely in the distribution of their gestational age--represented groups of different intrauterine growth rates." It is therefore a little difficult to understand what his later objections are, unless they stem from the fact that our 4 groups do not plot on the Denver, New York, and Baltimore curves the way that Dr. Valaes expected them to. However, there was no reason for him to expect that they should, since the method of constructing the Denver, New York, and Baltimore curves is the opposite of the method which we have adopted. We equalized the groups for birth weight, and the major variable which differentiated our groups was tile length of gestation, whereas in construction of the Denver, Baltimore, and New York curves, the method was to equalize for gestation and the major variable between the different percentile curves was that of birth weight. These methods provide for different comparisons. It should be noted that whether Dr. Valaes' arguments against our use of the term intrauterine growth are valid or not, they apply with equal, if not stronger, force to the Denver, New York, and Baltimore curves. It is therefore surprising that Dr. Valaes is satisfied to refer to the latter as "curves of intrauterine growth." The problem of the best way to utilize gestational age and birth weight--particularly the interaction between the two--in investigations of outcome of pregnancy and subsequent health and development of children, is indeed a very complex one. In a forthcoming paper we review the different classification systems which have been proposed, and we discuss in greater detail the issues and concepts that are involved. B~A J. VAN DEN ~ERG AND J. YERUSHAL1V[Y CHILD H E A L T H AND DEVELOPMENT STUDIES
3867 HOWE STREET OAKLAND, CALIF.
Letters to the editor
REFERENCES
1. Segal, S., Blair, A., and Roth, H.: The metabolism of galactose by patients with congenital galactosemia, Am. J. Med. 38: 62, 1965.
Intrauterine gro~vth rates and mortality
To the Editor: In their article Drs. B~a Van den Berg and J. Yerushalmyl presented a very clever and tricky method for dividing a neonatal population in such a way that the influence of gestational age on neonatal mortality could be examined independently of birth weight. They rightly claimed that the groups formed by their method--equalized for birth weight and differing widely in the distribution of their gestational age--represented groups of different intrauterine growth rates. Having started from this point and after a jump (in their reasoning), they ascribed all the observed differences in neonatal mortality and subsequent developnlent to the differences in intrauterine growth rates. Surely, the primary difference between their groups is gestational age. Thus their title, "The relationship of the rate of intrauterine growth of infants of low birth weight to mortality, morbitity, and congenital anomalies," and such statements as, "most striking is the extremely high risk of neonatal mortality for infants of fast intrauterine growth rate" are quite misleading. What they have actually shown is the extremely high risk of neonatal mortality for the more immature infants. Apart from this unwarranted substitution of growth rate for immaturity, another point requires comment. The authors chose to study the group of infants of birth weights between 1,600 to 2,500 grams. By cutting this piece from the natural population, they produced a completely artificial one. The effect is obvious if one plots the gestational age distribution of their 4 groups. How artificial this population is can best be demonstrated by the following example: The authors claimed that their group I (short gestation quartile) had a very fast intrauterine growth rate. This is true only in comparison to their other groups; in comparison to the natural popu-
The Journal of Pediatrics July 1967
2. Baker, L., Mellman, W. J., Tedesco, T. A., and Segal, S.: Galactosemia: Symptomatic and asymptomatic homozygotes in one Negro sibship, J. PEDIAT.68: 551, 1966.
lation, this was not at all a very fast-growing group. In the curves of intrauterine growth from Denver, 2 it lies near the seventy-fifth percentile. If we take for comparison the curves from New York City, a from which the authors derived their material, their group I (fast growing) lies at the fiftieth percentile, groups II and III are on the twenty-fifth and tenth percentile, respectively, and group IV is below the tenth percentile and corresponds therefore with the "intrauterine growth retardation" group. Hence, it seems that the conclusions of the authors are valid as far as the group with intrauterine growth retardation is concerned. In the other groups only the effects of gestational age were in fact observed. It seems that there is no method for separating the effects of growth rate from that of birth weight and gestational age. We have to compare either infants of the same birth weight but differing in gestational age or infants of the same gestational age but differing in birth weight. There is certainly great scope in studying the group of neonatal infants (apart from the wellknown cases born to diabetic or prediabetic mothers) with what appears to be a very fast intrauterine growth rate; this can be accomplished only by a prospective study with careful evaluation of each infant of this group with all the available criteria of maturity. No retrospective playing with numbers is going to solve the problem. Such a study is all the more important, as we cannot at present decide betwee~l the "rational" intrauterine growth curves of the Denver2 group and the unexpected curves--with wider distribution at early gestational ages--of New York, Baltimore, s or Newcastle? If such a group really exists, then these apparently unacceptable curves, with the almost flat ninetieth percentile, will be explained by the premature birth of most of the fast-growing fetuses, when ~hey approach the usual weight for a full-term infant. t am sure that Drs. Van den Berg and Yerushalmy will be able to provide this valuable information when they will analyze in more de-
Volume 71 Number 1
Letters to the editor
tail their data from the child health and development studies in Oakland. T. VALAES, M.D., D.C.H.
DIRECTOR~
"QUEENANNA
MARIA"
INSTITUTE OF CHILD HEALTH AGHIA SOPHIA CHILDREN'S HOSPITAL ATHENS, GREECE 6 0 9
REFERENCES 1. Van den Berg, B. J., and Yerushalmy, J.: The relationship of the rate of intrauterine growth of infants of low birth weight to mortality and congenital anomalies, J. PEDIAT. 69: 531, 1966. 2. Lubchenko, L. O., Honsman, C., Dressier, M., and Boyd, E.: Intrauterine growth as estimated from liveborn-birth-weight data at 24 to 42 weeks of gestation, Pediatrics 32: 793, 1963. 3. Battaglia, F. C., Frazier, T. M., and Hellegers, A. E.: Birth weight, gestational age, and pregnancy outcome, with special reference to highbirth-weight-low-gestational-age infant, Pediatrics 37: 417, 1966. 4. Neligan~ G.: A community study of the relationship between birth weight and gestational age, in Dawkins, M., and MacGregor, W. G., editors: Gestational age, size, and maturity, Clinic in Developmental Medicine, No. 19, 1965.
Reply To the Editor: We will not comment on such statements as "clever and tricky," and "played with numbers." When Dr. Valaes says, "Surely, the primary difference between their groups is gestational age," he forgot to add the crucial words "to attain their identical birth weight." Our main purpose was to investigate the interaction of birth weight and gestational age as they relate to mortality, morbidity, and congenital anomalies. In order to do that, we had the choice, in the words of Dr. Valaes, "to compare either infants of the same birth weight but differing in gestational age or infants of the same gestational age but differing in birth weight." We elected to use the first of these two methods because the relationship of birth weight to outcome far outweighs that of gestational age. We therefore eliminated the birth-weight factor by equalizing the groups for birth weight in order to bring into sharper focus the interaction with gestational age. The result, as we noted in the
15 1
paper, is that "It took the fetuses of the fourth gestational quartile 6 to 8 weeks longer than those of the first gestational quartile to attain the same birth weight. The 4 groups, therefore, differ primarily in their rate o[ intrauterine growth." Apparently Dr. Valses is in agreement with this view for he says, "They rightly claimed that the groups formed by their method--equalized for birth weight and differing widely in the distribution of their gestational age--represented groups of different intrauterine growth rates." It is therefore a little difficult to understand what his later objections are, unless they stem from the fact that our 4 groups do not plot on the Denver, New York, and Baltimore curves the way that Dr. Valaes expected them to. However, there was no reason for him to expect that they should, since the method of constructing the Denver, New York, and Baltimore curves is the opposite of the method which we have adopted. We equalized the groups for birth weight, and the major variable which differentiated our groups was tile length of gestation, whereas in construction of the Denver, Baltimore, and New York curves, the method was to equalize for gestation and the major variable between the different percentile curves was that of birth weight. These methods provide for different comparisons. It should be noted that whether Dr. Valaes' arguments against our use of the term intrauterine growth are valid or not, they apply with equal, if not stronger, force to the Denver, New York, and Baltimore curves. It is therefore surprising that Dr. Valaes is satisfied to refer to the latter as "curves of intrauterine growth." The problem of the best way to utilize gestational age and birth weight--particularly the interaction between the two--in investigations of outcome of pregnancy and subsequent health and development of children, is indeed a very complex one. In a forthcoming paper we review the different classification systems which have been proposed, and we discuss in greater detail the issues and concepts that are involved. B~A J. VAN DEN ~ERG AND J. YERUSHAL1V[Y CHILD H E A L T H AND DEVELOPMENT STUDIES
3867 HOWE STREET OAKLAND, CALIF.