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Use of nerve conduction velocity to determine gestational age in infants at risk and in very-low-birth-weight infants
Use of nerve conduction velocity to determine gestational age in infants at risk and in very-low-birth-weight infants Nerve conduction velocity was used to measure gestational age in at risk and very-low-birth-weight neonates. The method gave highly significant correlations with gestational estimates by the Dubowitz score and with confirmed maternal dates; 86 % o f the estimates agreed within two weeks with confirmed maternal dates. The method is valid in babies 30 weeks old or younger and is reproducible after the first postnatal week. There was no difference in babies small for gestational age. The technique was estimated to predict gestation with a standard deviation of + 1.14 weeks. (J PEDIATR 103.'109, 1983)
Geoffrey Miller, M.A., M.R.C.P., John Z. Heckmatt, M.B., Ch.B., M.R.C.P., Lilly M. S. Dubowitz, M.D., D.C.H., and Victor Dubowitz, M.D., Ph.D., F.R.C.P. London, England
THE ASSESSMENT OF GESTAT1ONAL AGE using a combination of neurologic and superficial criteria--the maturity estimate--has been widely used for several years. It gives a good prediction of gestational age, with 95% confidence limits of 2.0 weeks. ~Nerve conduction velocity has also been shown to be a useful method of assessing gestational age. 2-5 However, when compared with the maturity score, its accuracy has been less and errors inherent in the technique greater. 2'6 m greater variation in the NCV of infants small for gestational age has been found.2, 3This study was undertaken to reassess the validity of using the nerve conduction velocity to assess gestational age, particularly in an at risk population and in infants of very low birth weight.
mate, or from confirmed maternal dates when available, was 32.5 weeks (range 26 to 42 weeks). Their mean birth weight was 1620 gm (range 730 to 3810 gm). Forty-seven percent of the babies weighed less than 1500 gm. In 75 of the babies a maturity estimate (Dubowitz score) was performed independently. In 40 babies there were confirmed maternal dates as estimated from the last menstrual period. This date was accepted only from those mothers who had regular, normal cycles and had no bleeding subsequent to the last menstrual period. Fifty-one measurements were performed on these babies. Their mean gestation was 31.5 weeks (range 26 to 42 weeks). The mean birth weight was 1525 gm (range 730 to 3614 gm). Fifty-four percent weighed < 1500 gin.
MATERIALS AND METHODS The gestational age from the nerve conduction velocity was estimated by using the combined velocities of the ulnar and posterior tibial nerves,, as described by Moosa and Dubowitz, 7 because this method was shown to be more accurate than using a single nerve. One hundred measurements were performed on 79 infants in a neonatal intensive care unit. Their mean gestation from the maturity esti-
From the Department o f Paediatries and Neonatal Medicine, Royal Postgraduate Medical School, Hammersmith Hospital. Reprint requests: Prof. V. Dubowitz, Department of Paediatrics and Neonatal Medicine, Hammersmith Hospital, Ducane Rd., London W12 OHS, England.
GA-NCV
Gestational age from nerve conduction velocity [
The GA-NCV was compared with the maturity estimate and the confirmed maternal dates. The method used was to calculate the standard deviation of the mean of the sum of the differences between the two estimates, and to calculate the regression equation of GA-NCV on maturity estimate and confirmed dates. Multiple data points were taken from some patients, but this did not affect the conclusions. To demonstrate this, the same calculations were made without using the multiple data points, but by taking a random reading from those babies on whom more than one measurement was made. The Journal of P E D I A T R I C S
109
1 10
M i l l e r et al.
GANCV weeks
The Journal of Pediatrics July 1983
-..S
40-
00 9
30
9
/
s.e. :1.68 20
I 30
I MATURITYESTIMAE 40 weeks
Fig. 1. GA-NCV vs maturity estimate.
GANCV weeks 40
Difference (wk)
Number o f estimates (n = 100)
Percent
41 1 to 2 2 to 3 >3
66 17 10 7
66 17 10 7
Table lI. G A - N C V vs confirmed dates
r=O.87
I 20
Table I. G A - N C V vs maturity estimate
-
:,
"
Difference (wk)
Number o f estimates (n = 51)
41
34
1 to 2
10
2 to 3 >3
6 t
Percent
66.7 19.6 11.7 2.0
or more were divided into five groups: less than 32 weeks, 32 to 34 weeks, 35 to 38 weeks, older than 38 weeks, and small for gestationaI age. The proportion of babies in each of these groups whose G A - N C V differed by two weeks or more were compared. If confirmed maternal dates were known, these were used.
30-
5!!ii 20 -
20
I
30
I
40 CONFIRMEDDATES weeks
Fig. 2. GA-NCV vs confirmed dates.
The reproducibility of the method within 48 hours was assessed on I7 babies by calculating the regression equation of the first G A - N C V ( G A - N C V J on the second G A - N C V (GA-NCV2) and by estimating the standard deviation of the mean of the sum of the differences between GA-NCV~ and GA-NCV2. Dividing this standard deviation by ~ - g i v e s the standard deviation of one reading and a measurement of how well the technique predicts gestation. Nineteen babies had confirmed dates of 30 weeks or less. Their mean birth weight was 1040 gm (range 730 to 1280 gin). The G A - N C V was measured at either less than one week, one to two weeks, or more than two weeks. The mean of the sum of differences in weeks between G A - N C V and confirmed dates in these three .groups was then compared. The babies whose G A - N C V differed from the maturity estimate or from confirmed maternal dates by two weeks
RESULTS GA-NCV vs maturity estimate. The mean of the sum of the differences ( G A - N C V minus maturity estimate) was +1.22 weeks, with a standard deviation of 1.24 weeks. The regression equation of y ( G A - N C V ) on x (maturity estimate) gave a highly significant correlation of r = 0.87 at P < 0.001, with a standard error of the estimate about the regression line of 1.68 weeks (Fig. 1). Without using multiple data points the mean was - 0 . 1 2 weeks, with a standard deviation of 1.33 weeks, and r = 0.91 at P < 0.001, with a standard error of 1.58 weeks. GA-NCV vs confirmed maternal dates. The mean of the sum of the differences ( G A - N C V minus confirmed dates) was +1.1 week, with a standard deviation of 0.85 weeks. The regression equation of y ( G A - N C V ) on x (confirmed dates) gave a highly significant correlation of r = 0.94 at P < 0.001, with a standard error of the estimate about the regression line of 1.20 weeks (Fig. 2). Without using multiple data points the mean was +0.2 weeks, with a standard deviation of 1.38 weeks, and r = 0.92 at P;< 0.001, with a standard error of 1.41 weeks. Reproducibility w i t h i n 4 8 h o u r s . The mean of the sum of the differences (GA-NCV~ minus G A - N C V 0 for the G A - N C V on the same baby within 48 hours was +0.29 weeks, with a standard deviation of 1.61 weeks. From this it can be calculated that the technique predicts gestation with a standard deviation of -_+1.14 weeks. The regression equation of y ( G A - N C V ] ) on x (GA-NCV2) gave a
Volume 103 Number 1
Gestational age determined by nerve conduction velocity
Table IlL GA-NCV vs estimate when difference ~ 2 weeks Estimate (wk)
Total (n = 100)
GA-NCV vs estimate ~ 2 weeks
Ratio per group
<32 32 to 34 35 to 38 >38
31 36 21 12
6 9 3 6
0.19 0.25 0.14 0.50
GANCV weeks/
111
./-
Table IV, GA-NCV vs confirmed dates ~ 3 0 weeks Age when performed (wk)
Mean difference (w~)
Standard deviation (wk)
1.14
1.30
1 to2
1.67
1.10
>2
0
1.05
I 20 Number 7 6 6
highly significant correlation ofr = 0.86 at P < 0.001 (Fig. 3). Distribution of differences between estimates. Eightythree percent of GA-NCVs agreed with the maturity estimates within two weeks (Table I). When GA-NCV and confirmed maternal dates were compared, 86.3% of the estimates agreed within two weeks (Table II). GA-NCV vs estimates when difference was two weeks or more. In the infants less than 32 weeks, 32 to 34 weeks, 35 to 38 weeks, and older than 38 weeks, the proportion in whom the difference between GA-NCV and estimates was two weeks or more did not differ significantly (for 3 degrees of freedom Chi squared = 5.92, P < 0.15) ( T a b l e III). Although there were 50% in the group older than 38 weeks, this does not differ from the other age groups (for 1 degree of freedom Chi squared = 3.56, P > 0.05). Among the 15 babies who were small for gestational age, five had a GA-NCV that differed by two weeks or more from the estimates. This proportion did not differ significantly from that in the other age groups (for 1 degree of freedom Chi squared = 0.35, P = 0.8). GA-NCV vs certain dates of 30 weeks or less, The mean of the sum of the differences between these two estimates was +0.89 weeks, with a standard deviation of 1.36 weeks. By one-way analysis of variance there was no significant difference in those studied during the first or second weeks after birth or later (P > 0.05) (Table IV). DISCUSSION For routine use during the first postnatal week, the maturity estimate is still the most suitable single method for assessing gestational age. However, it has the disadvantages of subjectivity, reduced accuracy in infants 29 weeks or less, 8 and difficulty in applying to severely sick neonates
I 30
I 40
GANC~ weeks
Fig. 3. Reproducibility.
who are on ventilators or who have been subjected to surgery, and it cannot be applied after the first postnatal week. The GA-NCV is relatively simple to perform and has been shown to be unaffected by the common pathologic conditions affecting the neonate.2'9 It can also be used on sick neonates who are being ventilated or who are confined because of drainage tubes from surgical procedures. However, its accuracy has been questioned, one study 2 finding that the GA-NCV differed from estimates based on maternal dates by two or more weeks and that duplicate measures gave a standard deviation of _+2 weeks because of limitations inherent in the technique. Our study demonstrates that much g~eater accuracy can be achieved with the GA-NCV, with results within two weeks of maternal dates in 86% of measurements, and that there is good reproducibility. Gestation could be predicted with a standard deviation of ___1.14 weeks. Other studies have demonstrated the accuracy of GA-NCV, but not in a significant number of infants of low birth weight. 1~ ~ A greater variation in nerve conduction velocity in infants small for gestational age has been described?.7 We could not confirm this finding. Another study ~~also did not show a difference in term infants small for gestational age. The disadvantages of using the GA-NCV are the expense, the need for a constant environmental temperature (which is satisfied in a neonatal intensive care unit), and the care needed in measuring the distance between two points on a limb. There needs to be consistency between this measurement and the position of the limb. Even with this precaution, there is a 6.5% error between measured distance and the length of the nerve? The use of the technique has also been questioned in infants of very low birth weight of 30 weeks or less. Our study, however, demonstrates its validity in the very-
112
Miller et al.
low-birth-weight infant and has shown that the technique is reproducible on these babies if performed after the first postnatal week. This is an important advantage, as m a n y babies are transferred to intensive care units and estimates may not be performed until the infant's condition is stabilized. We thank Professor M.J.R. Healy (Professor of Medical Statistics) for help and advice with the statistical analyses, and Rhoda Stevenson for technical assistance. REFERENCES
1. Dubowitz LMS, Dubowitz V, Goldberg C: Clinical assessment of gestati0nal age in the newborn infant. J PEDIATR 77:1, 1970. 2. Moosa A, Dubowitz V: Assessment of gestational age in newborn infants: Nerve conduction velocity vs maturity score. Dev Med Child Neurol 14:290, 1972. 3. Schulte F J, Michaelis R, Linke 1, Nolte R: Motor nerve conduction in term, preterm and small for dates newborn infants. Pediatrics 42:17, 1968.
The Journal of Pediatrics July 1983
4. Koenigsberger MR, Curtin J, Lovelace RE: Motor conduction velocities as a measure of gestational age in premature infants: A study of multiple births--twins, triplets and quadruplets. Neurology 20:381, 1970. 5. Dubowffz V, Whittaker GF, Brown BH, Robinson A~ Nerve conduction velocity: An index of neurological maturity of the newborn infant. Dev Med Child Neurol 10:741, 1968. 6. Finnstrom O: Studies on maturity in newborn infants. Acta Pediatr Scand 61:33, 1972. 7. Moosa A, Dubowitz V: Postnatal maturation of peripheral nerves in preterm and fullterm infants. J PEDIATR 79:915, 1971. 8. Dubowitz LMS: Personal observations. 9. Blom S, Finnstrom O: Studies on maturity in newborn infants. V. Motor conduction velocity. Neuropaediatre 3:129, 1971. 10. Cruz Marrinez A, Perez Conde MC, Ferrer MT. Motor conduction velocity and H-reflex in infancy and childhood. Electromyogr Clin Neurophysiol 17:493, 1977. 11. Goeschen K, Pluta M, Rothe J, Saling E: Messung der motorischen nervenleitgeschwindkeit: Eine prazise methode zur Beurteilung der Reife des Neugeborenen. Z Geburtshife Perinat 185:144, 198l.
Geoffrey Miller, M.A., M.R.C.P., John Z. Heckmatt, M.B., Ch.B., M.R.C.P., Lilly M. S. Dubowitz, M.D., D.C.H., and Victor Dubowitz, M.D., Ph.D., F.R.C.P. London, England
THE ASSESSMENT OF GESTAT1ONAL AGE using a combination of neurologic and superficial criteria--the maturity estimate--has been widely used for several years. It gives a good prediction of gestational age, with 95% confidence limits of 2.0 weeks. ~Nerve conduction velocity has also been shown to be a useful method of assessing gestational age. 2-5 However, when compared with the maturity score, its accuracy has been less and errors inherent in the technique greater. 2'6 m greater variation in the NCV of infants small for gestational age has been found.2, 3This study was undertaken to reassess the validity of using the nerve conduction velocity to assess gestational age, particularly in an at risk population and in infants of very low birth weight.
mate, or from confirmed maternal dates when available, was 32.5 weeks (range 26 to 42 weeks). Their mean birth weight was 1620 gm (range 730 to 3810 gm). Forty-seven percent of the babies weighed less than 1500 gm. In 75 of the babies a maturity estimate (Dubowitz score) was performed independently. In 40 babies there were confirmed maternal dates as estimated from the last menstrual period. This date was accepted only from those mothers who had regular, normal cycles and had no bleeding subsequent to the last menstrual period. Fifty-one measurements were performed on these babies. Their mean gestation was 31.5 weeks (range 26 to 42 weeks). The mean birth weight was 1525 gm (range 730 to 3614 gm). Fifty-four percent weighed < 1500 gin.
MATERIALS AND METHODS The gestational age from the nerve conduction velocity was estimated by using the combined velocities of the ulnar and posterior tibial nerves,, as described by Moosa and Dubowitz, 7 because this method was shown to be more accurate than using a single nerve. One hundred measurements were performed on 79 infants in a neonatal intensive care unit. Their mean gestation from the maturity esti-
From the Department o f Paediatries and Neonatal Medicine, Royal Postgraduate Medical School, Hammersmith Hospital. Reprint requests: Prof. V. Dubowitz, Department of Paediatrics and Neonatal Medicine, Hammersmith Hospital, Ducane Rd., London W12 OHS, England.
GA-NCV
Gestational age from nerve conduction velocity [
The GA-NCV was compared with the maturity estimate and the confirmed maternal dates. The method used was to calculate the standard deviation of the mean of the sum of the differences between the two estimates, and to calculate the regression equation of GA-NCV on maturity estimate and confirmed dates. Multiple data points were taken from some patients, but this did not affect the conclusions. To demonstrate this, the same calculations were made without using the multiple data points, but by taking a random reading from those babies on whom more than one measurement was made. The Journal of P E D I A T R I C S
109
1 10
M i l l e r et al.
GANCV weeks
The Journal of Pediatrics July 1983
-..S
40-
00 9
30
9
/
s.e. :1.68 20
I 30
I MATURITYESTIMAE 40 weeks
Fig. 1. GA-NCV vs maturity estimate.
GANCV weeks 40
Difference (wk)
Number o f estimates (n = 100)
Percent
41 1 to 2 2 to 3 >3
66 17 10 7
66 17 10 7
Table lI. G A - N C V vs confirmed dates
r=O.87
I 20
Table I. G A - N C V vs maturity estimate
-
:,
"
Difference (wk)
Number o f estimates (n = 51)
41
34
1 to 2
10
2 to 3 >3
6 t
Percent
66.7 19.6 11.7 2.0
or more were divided into five groups: less than 32 weeks, 32 to 34 weeks, 35 to 38 weeks, older than 38 weeks, and small for gestationaI age. The proportion of babies in each of these groups whose G A - N C V differed by two weeks or more were compared. If confirmed maternal dates were known, these were used.
30-
5!!ii 20 -
20
I
30
I
40 CONFIRMEDDATES weeks
Fig. 2. GA-NCV vs confirmed dates.
The reproducibility of the method within 48 hours was assessed on I7 babies by calculating the regression equation of the first G A - N C V ( G A - N C V J on the second G A - N C V (GA-NCV2) and by estimating the standard deviation of the mean of the sum of the differences between GA-NCV~ and GA-NCV2. Dividing this standard deviation by ~ - g i v e s the standard deviation of one reading and a measurement of how well the technique predicts gestation. Nineteen babies had confirmed dates of 30 weeks or less. Their mean birth weight was 1040 gm (range 730 to 1280 gin). The G A - N C V was measured at either less than one week, one to two weeks, or more than two weeks. The mean of the sum of differences in weeks between G A - N C V and confirmed dates in these three .groups was then compared. The babies whose G A - N C V differed from the maturity estimate or from confirmed maternal dates by two weeks
RESULTS GA-NCV vs maturity estimate. The mean of the sum of the differences ( G A - N C V minus maturity estimate) was +1.22 weeks, with a standard deviation of 1.24 weeks. The regression equation of y ( G A - N C V ) on x (maturity estimate) gave a highly significant correlation of r = 0.87 at P < 0.001, with a standard error of the estimate about the regression line of 1.68 weeks (Fig. 1). Without using multiple data points the mean was - 0 . 1 2 weeks, with a standard deviation of 1.33 weeks, and r = 0.91 at P < 0.001, with a standard error of 1.58 weeks. GA-NCV vs confirmed maternal dates. The mean of the sum of the differences ( G A - N C V minus confirmed dates) was +1.1 week, with a standard deviation of 0.85 weeks. The regression equation of y ( G A - N C V ) on x (confirmed dates) gave a highly significant correlation of r = 0.94 at P < 0.001, with a standard error of the estimate about the regression line of 1.20 weeks (Fig. 2). Without using multiple data points the mean was +0.2 weeks, with a standard deviation of 1.38 weeks, and r = 0.92 at P;< 0.001, with a standard error of 1.41 weeks. Reproducibility w i t h i n 4 8 h o u r s . The mean of the sum of the differences (GA-NCV~ minus G A - N C V 0 for the G A - N C V on the same baby within 48 hours was +0.29 weeks, with a standard deviation of 1.61 weeks. From this it can be calculated that the technique predicts gestation with a standard deviation of -_+1.14 weeks. The regression equation of y ( G A - N C V ] ) on x (GA-NCV2) gave a
Volume 103 Number 1
Gestational age determined by nerve conduction velocity
Table IlL GA-NCV vs estimate when difference ~ 2 weeks Estimate (wk)
Total (n = 100)
GA-NCV vs estimate ~ 2 weeks
Ratio per group
<32 32 to 34 35 to 38 >38
31 36 21 12
6 9 3 6
0.19 0.25 0.14 0.50
GANCV weeks/
111
./-
Table IV, GA-NCV vs confirmed dates ~ 3 0 weeks Age when performed (wk)
Mean difference (w~)
Standard deviation (wk)
1.14
1.30
1 to2
1.67
1.10
>2
0
1.05
I 20 Number 7 6 6
highly significant correlation ofr = 0.86 at P < 0.001 (Fig. 3). Distribution of differences between estimates. Eightythree percent of GA-NCVs agreed with the maturity estimates within two weeks (Table I). When GA-NCV and confirmed maternal dates were compared, 86.3% of the estimates agreed within two weeks (Table II). GA-NCV vs estimates when difference was two weeks or more. In the infants less than 32 weeks, 32 to 34 weeks, 35 to 38 weeks, and older than 38 weeks, the proportion in whom the difference between GA-NCV and estimates was two weeks or more did not differ significantly (for 3 degrees of freedom Chi squared = 5.92, P < 0.15) ( T a b l e III). Although there were 50% in the group older than 38 weeks, this does not differ from the other age groups (for 1 degree of freedom Chi squared = 3.56, P > 0.05). Among the 15 babies who were small for gestational age, five had a GA-NCV that differed by two weeks or more from the estimates. This proportion did not differ significantly from that in the other age groups (for 1 degree of freedom Chi squared = 0.35, P = 0.8). GA-NCV vs certain dates of 30 weeks or less, The mean of the sum of the differences between these two estimates was +0.89 weeks, with a standard deviation of 1.36 weeks. By one-way analysis of variance there was no significant difference in those studied during the first or second weeks after birth or later (P > 0.05) (Table IV). DISCUSSION For routine use during the first postnatal week, the maturity estimate is still the most suitable single method for assessing gestational age. However, it has the disadvantages of subjectivity, reduced accuracy in infants 29 weeks or less, 8 and difficulty in applying to severely sick neonates
I 30
I 40
GANC~ weeks
Fig. 3. Reproducibility.
who are on ventilators or who have been subjected to surgery, and it cannot be applied after the first postnatal week. The GA-NCV is relatively simple to perform and has been shown to be unaffected by the common pathologic conditions affecting the neonate.2'9 It can also be used on sick neonates who are being ventilated or who are confined because of drainage tubes from surgical procedures. However, its accuracy has been questioned, one study 2 finding that the GA-NCV differed from estimates based on maternal dates by two or more weeks and that duplicate measures gave a standard deviation of _+2 weeks because of limitations inherent in the technique. Our study demonstrates that much g~eater accuracy can be achieved with the GA-NCV, with results within two weeks of maternal dates in 86% of measurements, and that there is good reproducibility. Gestation could be predicted with a standard deviation of ___1.14 weeks. Other studies have demonstrated the accuracy of GA-NCV, but not in a significant number of infants of low birth weight. 1~ ~ A greater variation in nerve conduction velocity in infants small for gestational age has been described?.7 We could not confirm this finding. Another study ~~also did not show a difference in term infants small for gestational age. The disadvantages of using the GA-NCV are the expense, the need for a constant environmental temperature (which is satisfied in a neonatal intensive care unit), and the care needed in measuring the distance between two points on a limb. There needs to be consistency between this measurement and the position of the limb. Even with this precaution, there is a 6.5% error between measured distance and the length of the nerve? The use of the technique has also been questioned in infants of very low birth weight of 30 weeks or less. Our study, however, demonstrates its validity in the very-
112
Miller et al.
low-birth-weight infant and has shown that the technique is reproducible on these babies if performed after the first postnatal week. This is an important advantage, as m a n y babies are transferred to intensive care units and estimates may not be performed until the infant's condition is stabilized. We thank Professor M.J.R. Healy (Professor of Medical Statistics) for help and advice with the statistical analyses, and Rhoda Stevenson for technical assistance. REFERENCES
1. Dubowitz LMS, Dubowitz V, Goldberg C: Clinical assessment of gestati0nal age in the newborn infant. J PEDIATR 77:1, 1970. 2. Moosa A, Dubowitz V: Assessment of gestational age in newborn infants: Nerve conduction velocity vs maturity score. Dev Med Child Neurol 14:290, 1972. 3. Schulte F J, Michaelis R, Linke 1, Nolte R: Motor nerve conduction in term, preterm and small for dates newborn infants. Pediatrics 42:17, 1968.
The Journal of Pediatrics July 1983
4. Koenigsberger MR, Curtin J, Lovelace RE: Motor conduction velocities as a measure of gestational age in premature infants: A study of multiple births--twins, triplets and quadruplets. Neurology 20:381, 1970. 5. Dubowffz V, Whittaker GF, Brown BH, Robinson A~ Nerve conduction velocity: An index of neurological maturity of the newborn infant. Dev Med Child Neurol 10:741, 1968. 6. Finnstrom O: Studies on maturity in newborn infants. Acta Pediatr Scand 61:33, 1972. 7. Moosa A, Dubowitz V: Postnatal maturation of peripheral nerves in preterm and fullterm infants. J PEDIATR 79:915, 1971. 8. Dubowitz LMS: Personal observations. 9. Blom S, Finnstrom O: Studies on maturity in newborn infants. V. Motor conduction velocity. Neuropaediatre 3:129, 1971. 10. Cruz Marrinez A, Perez Conde MC, Ferrer MT. Motor conduction velocity and H-reflex in infancy and childhood. Electromyogr Clin Neurophysiol 17:493, 1977. 11. Goeschen K, Pluta M, Rothe J, Saling E: Messung der motorischen nervenleitgeschwindkeit: Eine prazise methode zur Beurteilung der Reife des Neugeborenen. Z Geburtshife Perinat 185:144, 198l.