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2A

Skills: Gestational age and weight

Contents

Objectives

When you have completed this skills chapter you should be able to:

Assessing the gestational age

2-a Assessing gestational age by simple inspection

There are a number of easily observable clinical signs that can help you decide whether an infant is term or preterm:

  Term Preterm
Sucks well Yes No
Flexes arms and legs Yes No
Veins seen under skin No Yes
Nipple clearly seen Yes No
Palpable breast bud Yes No
Descended testes Yes No
Covered labia minora Yes No

Postterm infants can usually be recognised by their long finger nails. With experience, most preterm infants can be identified by their general appearance and behaviour on clinical inspection.

2-b Scoring gestational age

To obtain a more accurate idea of the gestational age, the Ballard scoring method can be used. The accuracy of the method depends on the experience of the examiner. With practice and careful attention to detail, the infant’s correct gestational age can be estimated with an accuracy of about 2 weeks. If the scored age is within 2 weeks of the gestational age suggested by the mother’s dates, then accept her dates as correct. However, if the scored age is more than 2 weeks higher or lower than the mother’s dates, then her dates are probably incorrect and the scored age should be used. The scored gestational age can also be used to decide whether the gestational age, determined by obstetric assessment, is correct or not.

Other scoring methods such as the Finnstrom method and the Dubowitz method can also be used.

2-c Use of the Ballard method

The Ballard scoring method uses both neurological features and external features. The descriptions given below describe how to assess the features illustrated in Figure 2A-1. Each feature is given a score and these individual scores are added up to give a final total score. This final total score can then be converted to an estimated gestational age by consulting the table in Figure 2A-1. Where possible, examine both the left and right sides of the body when doing the Ballard score and give the average score observed on either side. Half scores can be used. Note that some features have negative scores for very preterm infants.

2-d Neurological features

All 6 neurological features are assessed with the infant lying supine (the infant’s back on the bed). The infant should be awake but not crying.

Posture: Handle the infant and observe the position of the arms and legs. More mature infants (with a higher gestational age) have better flexion (tone) of their limbs.

Square window: Gently press on the back of the infant’s hand to push the palm towards the forearm. Observe the degree of wrist flexion. More mature infants have greater wrist flexion.

Arm recoil: Fully bend the arm at the elbow so that the infant’s hand reaches the shoulder, and keep it flexed for 5 seconds. Then fully extend the arm by pulling on the fingers. Release the hand as soon as the arm is fully extended and observe the degree of flexion at the elbow (recoil). Arm recoil is better in more mature infants. Note that a score of 1 is not given.

Popliteal angle: With your one hand hold the infant’s knee against the abdomen. With the index finger of the other hand gently push behind the infant’s ankle to bring the foot towards the face. Observe the angle formed behind the knee by the upper and lower legs (the popliteal angle). More mature infants have a smaller popliteal angle with less extension of the knee.

Scarf sign: Take the infant’s hand and gently pull the arm across the front of the chest and around the neck like a scarf. With your other hand gently press on the infant’s elbow to help the arm around the neck. In more mature infants the arm cannot be easily pulled across the chest.

Heel to ear: Hold the infant’s toes and gently pull the foot towards the ear. Allow the knee to slide down at the side of the abdomen. Unlike the illustration, the infant’s pelvis may be allowed to lift off the bed. Observe how close the heel can be pulled towards the ear. More mature infants have less flexion of the hips and, therefore, you cannot bring the heel towards the ear.

2-e External features

Six external features are examined. The infant has to be turned over to examine the amount of lanugo on the back. If the infant is too sick to be turned over, then the amount of lanugo is not scored.

Skin: Examine the skin over the front of the chest and abdomen, and also look at the limbs. More mature infants have thicker skins.

Lanugo: This is the fine, fluffy hair that is seen over the back of small infants. Except for very immature infants that have no lanugo, preterm infants have a lot of lanugo and this decreases with maturity.

Plantar creases: Use your thumbs to stretch the skin on the bottom of the infant’s foot. Only note definite creases and not very fine wrinkles, that disappear when the skin is stretched. More mature infants have more creases. To measure the length of the foot in very small infants place a ruler on the sole and measure the distance in mm from the back of the heel to the tip of the big toe.

Breast: Both the appearance of the breast and the size of the breast bud are considered. Palpate for the breast bud by gently feeling under the nipple with your index finger and thumb. More mature infants have a bigger areola and breast bud.

Ears and eyes: Both the shape and thickness of the external ear are considered. With increasing maturity the edge of the ear curls in. In addition, the cartilage in the ear thickens with maturity so that the ear springs back into the normal position after it is folded against the infant’s head. The eyelids separate with increasing maturity.

Genitalia: Male and female genitalia are scored differently. With maturity the testes descend in the male and the scrotum becomes wrinkled. In females the labia majora increase in size with maturity. Note that a score of 1 is not given.

Males:

Females:

Each separate criteria is given a score after examining that sign on the infant. These separate scores are then added together to give a total score. From the total score the estimated gestational age can be read off the table.

Measuring weight and head circumference

2-f Weighing an infant

The naked infant is weighed, to the nearest 10 g, on a scale. Usually a digital scale is used. If a spring scale is used, it should be standardised with a known weight every month. If possible, the infant should always be weighed on the same scale. The birth weight must be recorded on the infant record card.

See Figure 2A-2 for a weight for gestational age chart.

Note
Measurements made on a spring scale are called weight while measurements recorded with a balance scale are called mass. The result is the same and both are read in grams (g). In this programme all measurements are called weight.

2-g Measuring head circumference

The occipito-frontal head circumference is measured with a tape measure or a special plastic head circumference tape to the nearest 1 mm. The largest head circumference must be measured around the forehead and back of the occiput. Usually the head circumference is measured after delivery when the weight is recorded. However, the measurement of head circumference should be postponed for 24 hours if marked moulding or severe caput are present at birth as they may result in an incorrect reading. If possible, the head circumference should be recorded on the infant record card.

The crown-heel length is usually not measured routinely as this is very inaccurate unless a special measuring box is used. Infant length is measured only in special circumstances, e.g. when dwarfism is suspected or for research on growth.

Plotting weight and head circumference

2-h Plotting weight for gestational age

On the chart in Figure 2A-2 an infant’s birth weight of 3000 g and gestational age of 39 weeks have been recorded. Note that lines have been drawn from the given weight and gestational age. The weight for gestational age is recorded at the point where these 2 lines meet.

Practise plotting weight for gestational age on the above chart by recording the following infants’ weight and gestational age. Decide whether each infant is overweight, appropriate weight, or underweight for gestational age. Remember that the centile lines mark the outer limit of the normal (or appropriate) weight for gestational age.

  1. Weight 1500 g and gestational age 30 weeks.
  2. Weight 1500 g and gestational age 34.5 weeks.
  3. Weight 3950 g and gestational age 39 weeks.
  4. Weight 4000 g and gestational age 42.2 weeks.
  5. Weight 3000 g and gestational age 43 weeks.

2-i Plotting head circumference for gestational age

Practise plotting head circumference for gestational age by recording the following infants’ head circumference and gestational age on the chart in Figure 2A-3. Decide whether each infant’s head is large, appropriate or small for gestational age.

  1. Head circumference 27 cm and gestational age 29.5 weeks.
  2. Head circumference 25.5 cm and gestational age 29 weeks.
  3. Head circumference 30 cm and gestational age 32 weeks.
  4. Head circumference 30 cm and gestational age 35.7 weeks.
  5. Head circumference 36 cm and gestational age 38 weeks.

References

The Ballard scoring method – J Pediatr 1991; 119: 417–423.
Weight for gestational age chart – Acta Paediatr Scand Suppl 1985; 31: 180.
Head circumference for gestational age chart – Pediatr Res 1978; 12: 987.

Figure 2A-1: The Ballard scoring method

Figure 2A-1: The Ballard scoring method

Figure 2A-2: Weight for gestational age chart

Figure 2A-2: Weight for gestational age chart

Figure 2A-3: Head circumference for gestational age chart

Figure 2A-3: Head circumference for gestational age chart