Assessment of gestational age in newborn with birth-weight less than 2 kgSimin F Irani, NB Kumta, AR Bhide, Preeti Mehta
Department of Paediatrics, K.E.M. Hospital, and Seth G. S. Medical College, Parel, Bombay-400 012, India
Neurological study of 190 newborns with birth-weight less than 2 Kg. was done. These babies were between 29 weeks to 40 weeks gestation of which 29 were babies of less than 33 weeks gestation and 161 were between 34 weeks to 40 weeks gestational age.
Gestational age and birth-weight are the two most useful parameters for assessing maturity of the new-born. The latter alone is not enough because newborn babies with similar birth-weights but different gestational ages behave differently. Determination of the date of conception is generally not easy. Other parameters used for assessment of gestational age are affected by intra-uterine growth retardation and hence are not helpful. Neurological status of the newborn is not affected by intra-uterine growth retardation and is therefore helpful in assessment of maturity. The Committee on Perinatal Mortality  has recently (1974) recommended that gestational age be classified into Pre-Term, Term and Post-Term.
A standard neurological examination was carried out in 190 newborn babies weighing less than 2 kg. This was done approximately 11-2 hours after feeds if possible. The reflexes and the responses tested were the palmar and the plantar grasp withdrawal response, Moro's reflex, blink response, Galant response, abdominal reflexes, pupil size and reaction to light, glabellar tap reflex, traction response, neck righting reflex, cross extensor reflex and cremasteric reflex. The response was classified as given below:
1. Response present more or less constantly whatever the state or gestational age of the baby.
2. Response depending mainly on gestational age.
3. Those responses which were dependent on the state of arousal.
4. Variability from baby to baby and from time to time.
1. Response constantly present
The palmar and plantar grasp responses were present in all the babies, although in babies of less than 31 weeks the palmar grasp consisted of only the flexion of the fingers.
The withdrawal response elicited by stimulating the sole of the foot was present in 188 babies.
The Moro's reflex was well elicited in all babies, except in six where the abduction and extension phase was feeble and of low amplitude. These babies were less than 29 weeks' gestation.
The blink response was present in 188 babies and depended on the state of arousal. The Galant response (Lateral incurvation of the trunk on stimulation of lumbar skin) was present in 186 babies. The abdominal reflex was present in 185 babies and was best seen in a not moving or crying baby.
2. Response dependent on Gestational age:
Pupil size and reaction to light: It is difficult to observe this response in Preterm babies. The lids of the eye are kept open and response to `switched on' light, and contraction of the pupils noted. The response was invariably absent in 13 babies and they all were below 31 weeks gestation.
Glabellar Tap Reflex: This is a blink of the eyelids in response to a tap on the glabella. In very immature babies the grimacing of the face is not considered a positive response. This response appears between 33-34 weeks gestation. Out of 190 babies the response was present in 169 babies. [Table 1].
Traction response: A newborn when pulled up by the wrist from the prone position responds by flexing its elbows and bracing the shoulders and by flexing its neck thus raising the head. The response tends to appear between 33 and 36 weeks gestation but the head raising part appeared slightly earlier. The response was present if either component was present. This reflex was present in 150 out of 190 babies.
Neck Righting reflex: The baby will rotate its trunk in the same direction in which the head of the baby is turned.
This reflex appears between 34-37 weeks. This was elicitable in 155 out of 190 babies. [Table 1].
Crossed Extensor Reflex: This is elicited by passively extending one leg and stimulating the sole of the foot. The extension of the opposite leg is a positive response. This response is present after 32 weeks' gestation. However the time of appearance of the reflex is unpredictable. In this study it was present in 161 babies. See [Table 1].
Cremasteric reflex was tested in male babies only and out of 81 babies it was present in 62 babies. The response appears between 32-34 weeks and was constantly present after this. In "small for dates" babies the cremasteric reflexes were more frequently absent than present after 34 weeks. [Table 1].
In the present study babies of different weights were classified according to various gestational age as seen in [Table 2].
The various neurological examinations were done in all these babies and those with mean birth weight 1.124 kg. had all the neurological response difficult to elicit. Most of them however were below 33 weeks' gestation and absence of a particular reflex was necessarily due to functional immaturity of the reflex etc.
The assessment of maturity is based on changes in posture and muscle tone. Increasing muscle tone -is a prominent feature of advancing maturity, but since the increase is a gradual one and not a sudden change from absent to present as with some of the responses, the gestational age from degree of muscle tone in the present study was done with Posture, Recoil, Popliteal angle, Heel to Ear manoeuvre and Scarf's sign. Each of this sign was given a score according to Dubowitz.  Scoring system is as in [Table 3]. As gestational age increased the score for each criteria increased suggesting the increase in the muscle tone and partly also due to elasticity of the ligaments.
It has been observed that babies with same weight but different gestational ages have variable clinical problems. Not all low birth weight infants are the product of pregnancies that terminate early.  A good number are born at full term, and hence gestational age is considered as an important adjunct to birth weight. However underweight babies with short gestational age have a more difficult problem in adjusting to the extra-uterine environment than low birth weight babies with longer gestational age. According to certain authors gestational age information is helpful in the identification of newborn infants at high risk. 
St. Anne Dergassies  has presented evidence that neurological development is mainly dependent on gestational age, while being largely independent of postnatal age. As far as the neurological behaviour is concerned it matters little whether an infant with a conceptional age of 40 weeks reached this age in utero or spent a later portion of the time as a premature. Babies of 4.0 weeks behave identically neurologically whether or not they are born prematurely and whether they were of normal weight or underweight. Saint Anne Dergassies' assessment of maturity is based largely on the changes in posture and muscle tone and on the nature of crossed extensor reflex.  Electromyographic studies do show strong and long lasting myotonic flexor activity in a number of neonatal reflexes such as Moro, recoil of the forearms and traction responses, whereas the extensors receive only short lasting innervation during comparable movements. A great number of the so called "primitive reflexes" depend on the tonic myotactic reflex activity of the newborn. In the wakeful full term infant an extension of the forearm is followed by a rather quick recoil into the flexed position. In accordance with flexor preponderance tonic myotactic reflexes are more easily obtained in flexor than in extensor muscles. They are difficult to demonstrate in infants of less than 34 weeks' gestational age, yet are normally present by 36 weeks.  In the present study traction reflex was well elicited after 34 weeks' gestation.
Increasing muscle tone is a prominent feature of advancing maturity, but since the increase is a gradual one, the gestational age from degree of muscle tone tends to be subjective. This is seen with the popliteal angle measurement where the angle decreases with increasing gestational age, and this is due to increased muscle tone and partly due to elasticity of the ligaments. In the present study the popliteal angle was significantly decreased in babies with more than 36 weeks gestation, but in some full term babies there was a full range of movement.
A number of neonatal reflexes are especially useful in determining gestational age in normal newborns since their development proceeds in spurts at particular time periods during gestation 34 weeks the flexion and abduction portion of the Moro's response increases. In the present study a feeble abduction and extension phase was seen in six babies.
The pupil reactions are always absent before 29 weeks gestation, but the eye can detect light and shows blink response. The blink response in the present study was present in 188 babies, and depended on the state of arousal. The failure of pupil to react before 31 weeks is due to a pupillary membrane interfering with contraction of the iris and this disappears by 30 weeks' gestation. The cremasteric reflex is absent before 32 weeks and may be due to poor scrotal development and incomplete testicular descent. This reflex in "small for dates" babies depends on their body weight rather than the gestational age. The cremasteric reflex in the present study was present constantly after 34 weeks except in 8 "small for dates" babies.
The gestational age at which each reflex first appeared was uninfluenced by birth weight being the same in "small for dates" babies as in those having a normal birth weight for their gestational age (except the cremasteric reflexes). A significant difference in behaviour was demonstrated between babies of similar birth weight but different gestational ages for the pupil reaction, glabellar tap response, and traction reflexes. A reflex must have a relatively clearcut time of appearance which is constant in normal birth weight babies and small for dates babies. Five reflexes fulfilled the criteria - Pupil reaction to light (29-31 weeks), Traction response (33-36 weeks), Glabellar Tap Reflex (32-34 weeks), Neck righting reflex (34-37 weeks), Head turning to light (32-3u weeks). Other reflexes showed some relation to gestational age but were too in-, constant in behaviour to serve as indices of maturity [Figure 1].
[Table 1], [Table 2], [Table 3]