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Tiny human placentas grown in a dish are so close to the real thing that they can fool a pregnancy test into giving a positive result. The aim isn’t to develop a full-sized placenta, but to study why some pregnancies go wrong.

Most cells used in lab studies form a flat layer when grown in a dish. This unnatural environment means they don’t behave as they would when surrounded by other cells in the body.

In the past few years, we have found the right cues to coax cells of several tissue types into forming complex 3D structures, creating miniature organs known as organoids.

Ashley Moffett at the University of Cambridge and her team looked at the hormones and other signalling molecules released by the placenta and uterus, and worked out by trial and error which ones are needed to grow placental organoids in the lab.

The group took samples of human placentas from early abortions and broke the tissue apart. When they added the cells to clumps of a gel-like substance to help support a 3D structure, they could grow mini placentas just half a millimetre wide. The lack of a blood supply limited further growth.

The organoids produced various placental proteins and formed into finger-like projections characteristic of the placenta’s microstructure. The team also did tests to confirm that the cells were fetal in origin – as happens in pregnancy – not maternal.

After several days, the organoids were making a range of hormones, including one detected by pregnancy tests, human chorionic gonadotrophin. A test kit placed in their dish showed a “pregnant” result.

Moffett says further work will help us understand why some pregnancies lead to stillbirths, small babies and pre-eclampsia – a dangerous rise in blood pressure in a pregnant woman.