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The 2nd Trimester Ultrasound Scan - Part 3

What they are looking for (...Continued).

Several measurements are also taken during the scan.  These all assist in the confirmation of gestational age and in diagnosis of any possible abnormalities.

The length between the top of the head and the base of the spine can give a good indication of gestational age before the 15th week.  In this second scan, other measurements give a better indication.

Probably the most important measurement is the circumference of the abdomen.  This provides a very good indication of the actual size and weight of the foetus instead of simply the gestational age.  The chart below gives an indication of the expected abdominal circumference at each week.  The accepted upper and lower variation are about 9% of the quoted values:

Femur:  Another important measurement is the length of the femur bone.  This is the longest bone in the body and gives a clear indication of the growth (lengthwise) of the baby.  It is useful for ruling out dwarfism as well as in calculations for gestational age.  Again, the chart below gives only the average growth figures by week:

Head size:  The other main measurement is the distance across the head (from one side to the other). This is less reliable for calculating age after 15 weeks, but is still usually taken.

Other measurements that may be taken include the distance between the eyes, the distance from the eye sockets to the front tip of the face and the size of the nuchal skin fold (the thickness of the skin around the neck area), all of which can give an indication of trisomy in some form. 

Recently, techniques have been developed that can measure the accumulation of fluid (translucency) in the nuchal skin fold by use of ultrasound equipment.

Introduction

1. Doctor visits during the 1st trimester 

2. 1st trimester ultrasound appointments

3. Doctor visits during the 2nd trimester 

4. 2nd trimester ultrasound scan - Part 1 

5. 2nd trimester ultrasound scan - Part 2 

6. 2nd trimester ultrasound scan - Part 3 

7. Doctor visits during the 3rd trimester 

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