Police have begun an investigation after a placenta and medical equipment were found discarded in woods.
A dog walker made the gruesome discovery near Stroud in Gloucestershire, shortly after 7pm on Thursday.
Police said the woman was walking her dog along a track when the animal disturbed a plastic bag.
The materials have been examined by a midwife, but it is impossible at this stage to say whether the placenta is from a human or an animal, police said. If it is from a human, the placenta is suspected to be from the birth of a baby of about 20 weeks gestation.
Local maternity units have denied admitting a woman who has given birth to a baby of that age.
Detective Inspector Steve Bean said: “Our overall concern is that there may be a woman who has medical needs, and equally there may be a very premature baby requiring urgent medical attention.”