Scientists have developed models of human embryos using stem cells to study early human development, investigate medical issues that occur before birth, and understand why many pregnancies fail. These models are not created from egg and sperm, and therefore, cannot develop into babies. They provide a snapshot of what might be happening in the embryo during pregnancy, but they are not complete enough to be used for reproduction.

The use of these models sidesteps the ethical controversy associated with using real embryos for research. Several research groups, including teams from the United States and England, are involved in this work. Some of the latest models mimic an embryo after it has implanted in the uterus, a stage that is difficult to observe in real human embryos because they embed themselves into the uterus.

To create these models, scientists use a type of stem cell that can develop into many different types of cells or tissues in the body. These stem cells can be derived from embryos or reprogrammed from adult tissues. Some of these models even mirror the very beginnings of the placenta and yolk sac.

The models resemble human embryos nine to 14 days after fertilization. This allows scientists to ask questions about how human development occurs during these early stages, which are typically hidden within the mother’s body. Researchers can also study embryonic failure, developmental disorders, and pregnancy loss.

In the future, these human embryo models could be used to explore the effects of the environment and chemicals on early development. They could also be used to generate tissues for new medical treatments. Additionally, they could be used for testing drugs and exposing them to germs, experiments that cannot be conducted on pregnant individuals.

However, guidelines from the International Society for Stem Cell Research prohibit placing any human embryo model into either a human or non-human uterus. While there is a public misconception that these models could be used to create pregnancies, scientific barriers, such as the inability to develop a proper placenta, prevent this. As the field progresses, measures can be put in place to prevent misuse of these models. The primary ethical reason for not making the models complete is to avoid the controversy associated with embryo research.