Many women in rural Africa give birth at home without the help of a medical professional. If the baby is clearly ill or in distress, they know to go to a hospital or clinic immediately. But when babies are born prematurely — defined as before 37 weeks of gestation — it isn’t always obvious that their lives may be in danger.

According to a report on BBC News Health, over 80 percent of the deaths caused by complications from premature birth occur in Sub-saharan Africa and South Asia.

Often, the babies live for a few weeks but fail to thrive and subsequently die by four weeks of age. In rural areas of Tanzania, for example, 1 in 30 premature babies will die within four weeks of birth. Yet, some simple advice could have saved their lives if only their mothers had known it.

One simple way to recognize when a baby is too small is by measuring its footprint.

“There’s this grey area when the baby is between around 2.4kg (5lbs 5oz) and 2.1kg (4lbs 10oz) when the baby is more vulnerable to infection and other issues,” said Dr Joanna Schellenberg of the London School of Hygiene and Tropical Medicine. “But when a baby is born at home, there is no way of weighing them.”

Local volunteers like Mariam Ulaya visit homes of newborns with a laminated card showing pictures of two sizes of baby’s feet. By holding the newborn’s foot against the card, she can tell if the baby is in the danger zone. A baby’s foot size has been shown to be a good indication of its weight.

It’s best if the foot can be measured within two days of birth, when it is  most in danger. If the baby’s foot is in the danger range, the mother is given simple instructions on how to help the infant survive.

“If I’ve measured the child’s footprint and seen that the child is smaller than usual, then I instruct them to carry the child skin-to-skin so that the child can share and feel the mother’s warmth,” said Ulaya. “I also carry a small doll with me called Opendo. I use the doll to illustrate the proper way to breastfeed the child.”

A report by the World Health Organization states that employing these two methods can save up to 75 percent of premature newborns’ lives. It’s advice that would normally be given to a mother giving birth in a hospital.

To women in rural Africa, it’s advice that starts with a footprint.