But it illustrated a hallmark of Zernicka-Goetz's work from the beginning, says Lilianna Solnica-Krezel, a developmental biologist at Washington University in St. Success was far from guaranteed-mentors had warned Zernicka-Goetz away from the difficult project-and took years of testing different culture conditions with mouse embryos. That's almost twice as long as previously achieved, and it opens the way to studying a key phase of human development that had been inaccessible. This time her lab showed-in concert with researchers in New York City-that human embryos created by in vitro fertilization (IVF) could be kept developing in the lab for nearly 2 weeks, well past the point at which they would normally implant in the wall of the uterus. Two months later, she caught the attention of other embryologists and the wider world with another demonstration of embryos' resilience. Her child's health depended in part on a question that had already captivated her as a scientist, she recalls: How much flexibility does the developing embryo retain? Or, as she puts it, “What is the minimum number of normal cells for the fetus to be normal?” By the time of her pregnancy, her research had convinced her that individual cells are starting to go their separate ways when the embryo has just four cells-a stance that was and remains controversial. Zernicka-Goetz is not only a developmental biologist she has spent more than a decade working in mice to pinpoint when and how cells in an early embryo start to differentiate. Nature could hardly have come up with a more vivid irony.

The University of Cambridge professor's specialty, developmental biology, “was suddenly personal,” she says. A genetic test of the fetal portion of the placenta showed that roughly a third of the cells carried a serious abnormality: an extra copy of chromosome 2. In 2006, Magdalena Zernicka-Goetz was pregnant, unexpectedly, at age 42.

Pushing the Limits by Lynn Hagen