Stem cells are cells that haven’t differentiated yet. Although each of our body cells contain the exact same DNA as was present in the very first fertilized egg that each of us developed from, liver cells look and function very differently than nerve cells. This is because certain sections of DNA are turned off to form liver cells, while different sections of DNA are turned off to form nerve cells. Stem cells contain DNA that is all fully functional, accessible, open or "turned on".
Initially, stem cell research was controversial because it was publicized that cells from the bodies of aborted fetuses were being used to manufacture the stem cell cultures, and some "pro-life" people thought that this contributed to an increase in the number of abortions.
Scientists sometimes do take human body cells to start a cell culture. The most well known is the HeLa culture that came from the cervix of Henrietta Lacks in 1950, and even though Henrietta passed away in 1951, cells from her cervix are alive and still being used in experiments in university science labs across the globe.
Cells from aborted fetuses were used to start stem cell cultures, and although this did not cause an increase in the number of abortions, many people felt queasy about this.
Later, umbilical cord blood was used to start stem cell cultures, and parents were urged to save some of their baby’s cord blood—for a price, of course, just in case (e.g., in case the child comes down with leukemia, stem cells could be cultured to form a bone marrow transplant).
Still later, other sources of stem cells were discovered. Baby teeth are one such source. In fact, baby teeth saved for years, perhaps in the back of a drawer, contain enough DNA to begin a stem cell culture that could be used to save the life of that child decades later.
More recently, scientists have learned how to coax regular old cells into becoming stem cells again (i.e., scientists have figured out how to turn on those sections of DNA that were turned off during differentiation).
THE ANSWER TO YOUR QUESTION IS that there is no longer any need to use any part of an aborted fetus for this purpose, thus "pro-life" people are more chill about stem cell research now.