By Anne Mawathe
BBC News, Nairobi
Nancy Akoth is four months pregnant and like many women in her state has strange cravings.
Some women eat coal, gherkins or soap but Mrs Akoth craves soft stones, known in Kenya, where she lives, as "odowa".
"I just have this urge to eat these stones. I do very crazy things, I would even wake up at night and go looking for them," she told the BBC.
"I consulted my doctor and all he told me is that maybe I'm lacking iron and gave me medication on iron, but I still have the urge to eat those stones."
Luckily for Mrs Akoth, she is not alone in craving stones and they are easily found on sale in Nairobi's sprawling Gikomba market.
Among the fish-mongers and dealers in second-hand goods who flock to the market are traders who specialise in odowa.
Stone-seller Stephen Ndirangu unsurprisingly says women are his main customers.
"Most of them buy the stones to go and sell them to women who are pregnant," Mr Ndirangu says.
He says he sells one 90kg sack for about $6.
Although they are stones, they are too soft to break the teeth of Mrs Akoth and her fellow cravers.
Nutritionist Alice Ndong says the stones have a bland taste.
"It's a pleasant taste. It doesn't have a tangy flavour or a salty or a sugary flavour. It's a bit like eating flour," she told the BBC.
I cannot do without it
She says that because of their abrasive nature, the stones actually clean the teeth as the stone is chewed and the finer particles pass through the mouth.
However, she warns this should not be used as an excuse to eat the stones as the habit can also have harmful consequences.
"If somebody eats those stones and they don't take enough water, then they will actually get severe constipationů It can actually be very dangerous," she says.
"It can actually cause things like kidney damage and liver damage, if you don't take enough fluid because it will form a mass that cannot be excreted."
"When you eat these stones, it's like eating metal. The particles - because it's not food - are not digested as finely as fruits or vegetables," she says.
The phenomenon of craving non-food items like soil or soft stones is referred to as pica, a Latin word for magpie, the bird notorious for eating almost anything.
Researchers from the University of Nigeria interviewed 1,071 pregnant women attending a prenatal clinic at the Pumwani Maternity Hospital in Nairobi.
At least 800 of those interviewed said they ate soil, stones and other non-food items during their pregnancy.
But it is not only those who are pregnant who indulge in this habit.
Sylvia Moi still finds the soft stones irresistible, 14 years after she gave birth.
"I cannot do without it... Walking without it makes me feel bad, as if I'm lacking something [or] I'm hungry," Mrs Moi says.
She says she would like to quit the habit but just cannot stop herself.
"When you eat it you look awkward, people think: 'What is it that you lack in you that makes you eat that awkward stone,'" she says.
Experts say that the craving to eat odowa is largely due to a deficiency of vital minerals, like calcium, in the body.
"Unfortunately, these stones don't offer a lot of calcium. They offer some other forms of minerals like magnesium but not much calcium," says Mrs Ndong.
Research shows that these habits have negative side-effects on the women's health, ranging from parasitic infestations, anaemia and intestinal complications
"The problem with these stones is sometimes they're not hygienic. I remember up-country I've seen people just go somewhere, dig up and maybe people urinate in that spot," she says.
Experts warn pregnant women and others who enjoy eating odowa to try to ignore these cravings for the sake of their health.
The researchers say that the women are better off eating a balanced diet, than remaining hooked to the myth that their changing bodies need soft stones and soil.