By Rabbi Jonathan Siger
First things first, even that which has the potential to become a human being is considered sacred; and even the wasting of semen is forbidden by the Torah. All life belongs to the Source of Life, and must be respected; and developing life is no different in this regard.
It is also clear that until birth, a fetus is not a 'person.' Torah instructs that if a pregnant mother is caught between two men who are fighting and as a result she suffers a miscarriage, the responsible person is punished with a fine. If the mother herself is killed, it is treated as a murder. (Exodus 21:22-23)
The Mishna teaches that if a woman miscarries prior to forty days, it is not considered a 'real' pregnancy. (M. Niddah 30a) Prior to 40 days, an embryo is considered "like water" (B. Yebamot 69b).
At the same time, we are allowed to violate Shabbat to save a fetus, and Halachic authorities are consistent in forbidding the intentional destruction of a fetus, even within the first 40 days unless a life-threatening situation demands it.
In point of fact, when we speak of embryonic stem cells, we are speaking of interior cells from within a blastocyst--a stage of development that occurs four or five days after conception. The cells at this stage are pluripotent--they can become any type of cell the body needs and for that reason their healing and regenerative promise is virtually limitless.
Now, so far, we can't justify the creation of embryos just for the sake of harvesting these stem cells. At least, there aren't any traditional authorities that have been willing to rule that this is an acceptable practice.
However, there are plenty of stem cells to be had. And we didn't have to create life just to harvest them.
These cells exist today as the by-product of fertility treatments. In-Vitro Fertilization procedures often result in many more zygotes than can be used, and many authorities have allowed these 'leftovers' to be discarded for a number of reasons, including the fact that they have, without a host uterus, no chance for further development and no potential to become a human being.
Cells from a embryo will otherwise be discarded are certainly considered usable in the development of medicines, therapies and methods by which lives can be saved. In fact, it is a far better use for these cells than simply throwing them away. While they may not have allowed for the fulfillment of the Mitzvah of Reproduction, they can be used in the Mitzvah of Saving Lives.
For further reading:
Biomedical Ethics and Jewish Law, Fred Rosner, MD, Ktav Publishing
Rabbi Jonathan Siger is the executive director of Central Florida Hillel.