Expectant mother Mrs Campbell (Rose Wardlaw) is at the clinic waiting to be seen by a midwife when she is hit by a powerful craving. Guiltily, she removes a bundle of cloth from her handbag, unwraps a lump of coal and takes a bite.
As Nurse Lucille Anderson (Leonie Elliott) explains when she catches sight of the coal dust smudged on her pregnant patient's cheek, Mrs Campbell is suffering from pica. And it's nothing to be ashamed about.
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Pica is a name for a disorder characterised by an appetite for substances without nutritional value – that is, it makes you want to eat things that are not food.
The affected person will feel a powerful compulsion to eat substances including paper, drywall, coal, metal, stones, soil, glass, chalk, ice or cigarette butts. Some of these forms of pica have specific names like acuphagia (sharp objects) cautopyreiophagia (burnt matches) and geophagia (dirt, soil, clay).
This is more than just babies wanting to put everything in their mouths, because pica is diagnosed when eating such objects is "developmentally inappropriate". It is most commonly seen in small children, people with developmental disabilities, and – as in Call the Midwife – in pregnant women.
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Is pica dangerous?
The effects of pica can be severe, especially when the sufferer is eating sharp substances such as glass or dirt that could contain parasites or anything containing lead.
There can be the risk of intestinal obstruction and tearing of the stomach, and filling up on non-foods can lead to nutritional problems.
Pregnant women with pica are generally urged to talk to their midwives and medical professionals to see how the craving can be managed and ensure they are staying healthy.
Why would a pregnant woman crave coal?
There is no widespread agreement on what leads to pica.
In some cases, mineral deficiencies have been associated with pica: the sufferer may be anaemic or lacking in zinc, causing the craving – and some have suggested the desire for coal in particular could be linked to an iron deficiency. But the extent to which this is a factor is disputed: the medical evidence is not available, and there is little to support the idea that nutrients can be absorbed from these non-food sources.
Certainly, for pregnant women, cravings are normal. Women go through extreme hormonal and physical changes during this period and their sense of taste and smell can be greatly impacted.
For some people it's pickles and peanut butter, for some people it's chilli peppers with marmite, and for others it's a nice lump of coal.
In 2008, the BBC reported: "Pregnancy cravings are much more common among modern women than previous generations, a survey suggests. Three-quarters of those surveyed experienced a craving, compared to just 30% five decades ago.
"It found a third of cravings were not for food, but items such as coal, soap, toothpaste and sponges."
Fiona Ford, of the University of Sheffield's Centre for Pregnancy Nutrition, told the BBC that cravings for non-food items were probably often related to smell and texture, not taste.