Standard, Population & Customised fetal size charts 9 – gender and parity

Customisation by fetal gender

After birth, male and female babies differ in size, and gender specific charts are widely used. This is reasonable since gender specific differences in mortality or morbidity are unrelated to fetal size.  On the contrary, male babies tend to be larger but have higher rates of mortality and morbidity, so gender specific charts are likely to better identify pathology.

There are also fetal gender differences in size (click here), so in theory gender specific charts might also be appropriate in utero.  However, in practice the gender differences are rather small, and because of practical difficulties – the ultrasonographer might be uncertain, or the parents might not want to know – gender specific fetal charts tend to be limited to specialist uses (click here).

Customisation by maternal parity

Birth weight and size increase with parity, at least over the first few pregnancies, but this is not independent of pathology. First babies are smaller than subsequent ones but have a higher mortality, partly because pre-eclampsia is common in first pregnancies, and partly because first labours tend to be longer. To a lesser degree, at least in the developed world, some of the increase in birth weight with increasing parity is related to increasing maternal weight. Again this is not benign. For these reasons customisation on maternal parity is unwise.

Tomorrow another technical digression. This time on estimated fetal weight (click here).

Jim Thornton