February 29, 2024


Scientists are calling for a review of the 14-day rule on embryo research, saying that extending the limit could help uncover the causes of recurrent miscarriage and congenital conditions.

Until now, scientists studying the earliest stages of life have been limited to growing embryos up to the equivalent of 14 days of development. They can then pick up the path of development a few weeks later, on pregnancy scans and from material donated from terminations.

But it leaves behind a “black box” period of two to about four weeks of development that has never been directly studied and that scientists say could hold the key to improving fertility treatments and understanding a variety of birth defects.

With a revision of fertility laws on the horizon and rapid scientific advances underway, scientists are calling for a review of the 14-day rule.

Dr Peter Rugg-Gunn, from the Babraham Institute in Cambridge, said: “The period of two weeks to four weeks has been labeled the black box of embryo development. There is currently no practical way to study it, so our knowledge is really limited. Studying embryos beyond the 14-day limit may have benefits for patients. The sooner this could be allowed, the sooner patients in the UK can benefit.”

Potential benefits include finding the causes of implantation failure, where the embryo does not implant in the uterine lining, which causes miscarriage, and the origin of congenital heart defects, which affect about one in 100 births and are estimated to account for about 40% of prenatal deaths.

“My view is that it’s important for people to understand what the benefits could be,” added Rugg-Gunn, who stopped short of directly calling for an extension of the limit as part of comprehensive proposals by the Human Fertilization and Embryology Authority (HFEA) to modernize the law.

Chart showing stages of embryo development available and unavailable for research

The UK’s 14-day rule was first proposed in the 1984 Warnock Report on the Ethics and Regulation of IVF technology, and has been law since 1990. It prohibits the cultivation of embryos after 14 days of development or before the formation of the primitive streak (which sets the body’s axis) and was intended to balance the potential medical benefits of research with the special status of the human embryo.

However, in 1990 the limit was theoretical because scientists could not maintain embryo development in the laboratory for more than a few days. In the last five years this has changed and a growing number of laboratories around the world are able to closely replicate development to the legal limit.

“We are now at the point where technically these experiments are probably possible,” Rugg-Gunn said. “There is a very high probability that if research can continue, the new knowledge will have benefits for health, especially for understanding the causes of recurrent miscarriage.”

Just beyond day 14, gastrulation occurs, an important step in which the embryo changes from a simple ball of cells into three distinct layers of tissue that establish a primitive body plan. “This is one of the most important steps in all development, but it has never been studied or visualized before,” Rugg-Gunn said.

Implantation of the embryo into the uterine lining (endometrium) occurs between days six and 12, but the process continues – and can go wrong – after day 14, and is considered a common reason why IVF treatment does not work.

Prof Kathy Niakan, a developmental biologist at the University of Cambridge, said: “There is an immunological interaction that is really unique at that time of pregnancy. There is this very interesting question of why, in some cases, the maternal cells and the fetal cells cannot coexist without some kind of attack or failure.”

During the third week, cells continue to differentiate and the first heart cells are formed. A large proportion of congenital heart conditions are thought to arise during this very early developmental period. Between day 21 and 28, the neural tube (the embryonic forerunner of the central nervous system) form and close. Spina bifida is caused by a failure of the neural tube to close properly, but the exact steps have not been directly observed. From around four weeks, scientists begin to gain insights into development from pregnancy scans and embryos donated from terminations.

Some argue that scientists may be overstating the potential clinical benefits of growing embryos after 14 days and question whether the ethical arguments supporting the legal limit have really changed.

“Limits are meaningless if they don’t actually prevent you from doing something,” said Prof Anna Smajdor, a philosopher at the University of Oslo. “Now [scientists] can do these things, they don’t want to be limited. The risk is that it makes a mockery of the idea that these are moral cut-offs that are the product of honest, moral consultation with scientists.”

It is now clearer than it was in 1990 that an embryo does not have a functional nervous system at 28 days, but Smajdor said the ethics involved “are not necessarily reducible to ‘does it feel pain?’ Even without a religious perspective, it is possible to think embryos have a moral value because they have the potential to become human beings. There is a symbolic component to it.”

Others suggest that the onus has shifted with scientific progress. “Human embryos … are a rare and precious resource,” said Sarah Norcross, chief executive of the charity Progress Educational Trust. “Is it right that scientists are legally required to stop studying these embryos in the lab after 14 days, when we could learn so much more from them, and when we could use this knowledge to better understand pregnancy loss and disease?”

Many believe with reforms to the law on the way, it is at least time to reopen the debate. “Having the discussion doesn’t mean the rule will change,” Niakan said. “It means having an open, two-way dialogue about what can be gained, what the potential risks are and asking how we feel about it.”



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