The earlier the stage of the fetus’s gestational development, the more room there is for leniency in allowing an abortion. Conversely, the more developed the fetus is, the higher the degree of life it exhibits, and even the most lenient opinions would permit abortion only in more drastic cases.
Until forty days have passed since conception, the fetus’s organs have not yet begun to form, and it does not yet even have the status of a fetus. Consequently, if a woman aborts or miscarries her fetus within the first forty days, if she subsequently bears a son, he has the status of “firstborn,” as the earlier fetus is considered “mere water” (Yevamot 69b). Based on this, even some of those who adopt the stringent view permit abortion during the first forty days in a case of great need.
From day 41, since the fetus’s organs have begun to take shape, those who adopt the restrictive approach maintain that abortion may be performed only if the mother’s life may be in danger. Nevertheless, some of those who adopt the restrictive approach maintain that since the woman’s pregnancy is not considered discernible until three months have passed, in cases of great need, abortion can be permitted (several Aḥaronim cited by Yabi’a Omer, EH 4:1).
In contrast, according to those who take the permissive approach, since abortion is not prohibited as a form of murder, it is permitted even after the end of the first trimester in cases of great need. However, the more developed the fetus is, the more compelling a reason is required to permit aborting it. Therefore, couples are encouraged to perform any necessary testing as early as possible, so that if an abortion is necessary it can be done as soon as possible (Tzitz Eliezer 9:51:3; Amud Ha-yemini §32).
Once a fetus is viable, that is, that it could survive on its own if it were born, it is almost impossible to permit abortion, even according to those who take the permissive approach. However, if it is clear that the fetus would not live for more than thirty days even if it were born naturally, then in certain cases some of those who rule permissively would permit abortion.
When pregnancy jeopardizes the mother’s life, all poskim permit abortion; even if labor has already begun, we kill the fetus to save the mother; but once the baby’s head or most of its body has emerged, we do not hurt it, for it is considered a human being, and we do not kill one person to save another (Ohalot 7:6).
Nevertheless, the Talmud states that a kohen’s daughter carrying the baby of a non-kohen (to whom she is not married) is still considered part of the kohen’s household and may therefore still eat teruma during the first forty days of pregnancy, since the fetus is considered “mere water.” However, once the fetus is older than that, it has status, and she may no longer eat teruma (Yevamot 69b).
The laws that determine who is a firstborn reflect a similar principle. An oldest son, born after a previous pregnancy ended with an abortion or miscarriage within forty days of conception, has the status of a firstborn, for it is only after forty days that a fetus becomes substantial and begins to develop organs. Before this, it is insubstantial (SA YD 305:23). Doctors today know that in a normal pregnancy, the fetus’s organs have not begun to form by day forty; they only begin to take shape after day 42. It is important to note that doctors normally calculate the beginning of a pregnancy from the day the woman got her last period, though conception generally occurs at least fourteen days later, at the time of ovulation. In contrast, the forty days are counted from conception, that is, the union of the sperm and ovum. When an abortion or miscarriage takes place more than forty days after conception, but the fetus stopped developing beforehand, the status of the subsequent child hinges on the fetus’s developmental stage. If it had not yet reached the development expected by day 41, a subsequent firstborn son must have a pidyon ha-ben (redemption of the firstborn). If there is uncertainty, the son is redeemed, but without a berakha.
In practice, since a fetus is considered “mere water” for the first forty days, those who are lenient and allow abortion in cases of great need would be lenient for less pressing needs in that early stage. We see this clearly in Responsa Be-ohala Shel Torah (1:115), where R. Ariel writes that an unmarried pregnant woman who finds it too difficult to give her child up for adoption may abort her pregnancy until day forty (likewise, Responsa Bnei Banim 3:38). Moreover, some poskim who tend to be restrictive about abortion in general, such as R. Shlomo Zalman Auerbach, are more permissive during the first forty days in cases of need (Nishmat Avraham, ḤM 425:1 n. 4, writing about Tay-Sachs). Nevertheless, some are as restrictive about abortion before the fortieth day as they are about after. In their view, the primary consideration is that the fetus is a potential human being on whose behalf we permit desecration of Shabbat (R. Unterman; Igrot Moshe, ḤM 2:69 is inclined to agree with him). See below, section 11 and n. 10.
Ḥayim Ozer Grodzinski writes that non-Jews do not commit a capital crime by aborting within the first forty days of a pregnancy (implying that there is no prohibition for them to do so), while there is a rabbinic prohibition for Jews to do so (Responsa Aḥiezer 3:65, at the end). R. Weinberg writes this as well (Seridei Esh, ḤM 162:22). Torat Ḥesed, EH 42:33 agrees regarding non-Jews.
. There would seem to be no differentiation within the period from day forty until the fetus is viable, and indeed, thus states Ḥavot Ya’ir §31. However, we must acknowledge that even those who adopt the permissive approach find it more difficult to permit abortion as the pregnancy advances, because the ḥavala and hashḥata are greater. Therefore, even after forty days have passed, and even according to the permissive approach, it is necessary to expedite medical tests to the degree possible, so that if an abortion is necessary, it will be done as early as possible.
According to those who adopt the permissive approach, even once the fetus has become viable, killing it is not considered murder but rather ḥavala or hashḥata. Presumably, then, for a very great need it is possible to permit its abortion. As we have seen, the Talmud (Arakhin 7a) states that if a pregnant mother is sentenced to death by beit din, they kill the fetus before executing her in order to avoid possible post-mortem degradation. Only if the woman is already in labor do we wait until she gives birth before executing her. However, in practice, even those who are more permissive regarding abortion do not generally permit it once the fetus is viable, since it is closer to being considered a human life. This is also implied by R. Waldenberg, the foremost proponent of the permissive school of thought, who states that from the end of the seventh month and onwards, one must be stringent (Tzitz Eliezer 13:102:5-6). We have also seen (beginning of n. 5) that even some who maintain that abortion is not murder still prohibit abortion unless carrying the pregnancy to term would be life-threatening (Shevet Ha-Levi 7:208). Similarly, even those who are generally lenient are stringent once the fetus is viable. (However, in Tzitz Eliezer 9:51:3 [p. 239, summary point 14], R. Waldenberg writes that the prohibition is most severe when the woman is in labor. This implies that when she is not yet in labor, it is possible to be lenient in very difficult cases even late in pregnancy, as explained above.)