One size fits all (Image: Reuters/Carlos Garcia Rawlins)

Babies come in all shapes and sizes – or so you might imagine. But according to new international growth charts, a healthy fetus or newborn baby should look broadly the same, regardless of ethnicity or its mother’s size. The charts have been developed to help identify signs of under-nutrition, stunting, wasting or obesity at the earliest possible stage of development, so that they can be addressed.

Yet the new charts go against recommended practice in many countries – including the UK – where midwives or sonographers assessing fetal growth customise their interpretation to take the parents’ ethnicity or stature into account.

“There are currently over 100 different charts that are used to describe the growth of babies,” says Stephen Kennedy at the University of Oxford, who led some of the research. “What that means is that a woman can go to hospital, have her fetus measured and be told it is normal, but if she went down the road and saw someone else, she might be told her baby isn’t growing well. This lack of standardisation increases anxiety, and can lead to intervention where intervention may not be required.”


Standard growth

To combat this problem, the INTERGROWTH-21st Project pooled data from thousands of healthy, well-fed mothers from the US, UK, India, China, Brazil, Oman, Kenya and Italy, using identical equipment and methods to take regular measurements of their fetuses and newborn babies. These were then used to plot standard growth charts, representing what a fetus’s abdominal circumference, length of thigh bone and head measurements should look like under optimal conditions. For newborns, they plotted a baby’s weight, length and head circumference.

There were some regional differences; for instance, the average birthweight among Indian babies was 2.9 kilograms, versus 3.5 kilograms in the UK. However, Kennedy says the pooling of such data means that the growth curves are representative of healthy babies around the world.

“Provided the woman is healthy, all babies grow in a similar way and achieve a similar size at birth, irrespective of a woman’s skin colour, her ethnicity or where she lives,” Kennedy says.

Their approach is identical to the one used by the World Health Organization to draw up the charts that are used to assess children’s growth from birth until the age of 5. Yet some feel that a “one-size-fits-all” standard for fetal growth goes against a body of evidence suggesting that ultrasound scans should be customised to take maternal characteristics such as ethnicity into account.

“This is a backward step and it just causes confusion,” says Jason Gardosi, director of the Perinatal Institute in Birmingham, UK. “It means that smallish ladies, or ladies of different ethnicities who are absolutely normal and healthy, could be told that their baby is pathologically small because of a one-size-fits-all standard.”

Different rules

Kennedy acknowledges that there are genetic differences between babies, but says these have a far smaller effect than environmental factors. “Paediatricians have no problem with the idea of international standard for optimal growth from birth until the age of 5,” he says. “All we have done is fill in the gap that exists from pregnancy to birth. Mothers need to ask why their babies are monitored in utero using customised charts and ex utero using international WHO charts that apply to all babies worldwide.”

He adds that the medical community will need to decide for itself how to implement the new growth charts, and what should happen if a baby deviates from the norm.

“Gardosi’s point is valid, but tools like these are needed for public health reasons, for instance, when following the growth development of premature infants,” says Ola Didrik Saugstad, a paediatrician at the University of Oslo in Norway. “They will never be perfect because the factors contributing to babies’ growth are complicated. Doctors assessing a baby or fetus who deviates from the standard will need to fully explore the reasons behind it before concluding that there is something wrong.”

Journal references: The Lancet, DOI: 10.1016/S0140-6736(14)60932-6,10.1016/S0140-6736(14)61490-2