It is not written into the Constitution as such. This issue has always revolved around the question of whether a fetus which is a part of the mother's body is a human being. Human beings have specific rights under the Constitution. There is general agreement that biologically, before a baby is born it does not have a life independent of the mother. The idea that human life begins at conception is a religious concept. Secular society has accepted that at some point between conception and birth there is a transition from fetus to human being even though that human being may not yet be independent of it's mother. The point at which this occurs can not be precisely determined in actual practice. Therefore the Court has accepted that the determining factor is that point in a pregnancy before which no birth has resulted in a child that could survive outside it's mother's womb. To the majority of people this criterion is logical and acceptable, and no abortions are therefore legal, except in extraordinary cases (vide infra), before the time at which birth can result in a surviving human being.
An exception is drawn where there arises a conflict between the Mother's life and the life of what could potentially become a surviving neonate. Here it has been decided that the mother's life is superior to the potential neonate.
This is, as I understand is the law as it stands. Law's are subject to change however. The U.S. Constitution Reads :
Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof. This is generally referred to as the "Establishment Clause." Much depends, I would think, on how the Court interprets : "...Respecting an establishment of Religion." One meaning particular meaning there has never been any controversy over, viz., Congress shall not establish any particular religion as the Nation's Religion, as in a theocracy for example. But other readings are not as clear.
Here, in part, is what the Cornell Law School say about the Establishment clause:
The
First Amendment's Establishment Clause prohibits the government from making any law “respecting an establishment of religion.” This clause not only forbids the government from establishing an official religion,
but [it] also prohibits government actions that unduly favor one religion over another. It also prohibits the government from unduly preferring religion over non-religion, or non-religion over religion.
Although some government action implicating religion is permissible, and indeed unavoidable, it is not clear just how much the Establishment Clause tolerates. In the past, the
Supreme Court has permitted religious invocations to open legislative session,
public funds to be used for private religious school bussing and textbooks, and
university funds to be used to print and publish student religious groups' publications. Conversely, the
Court has ruled against some overtly religious displays at courthouses,
state funding supplementing teacher salaries at religious schools, and some overly religious holiday decorations on public land.
These actions described above and implicating religion do not unduly burden a non-religious person and the Courts have allowed them as a part of our right to free exercise of religion. When it comes to the matter of abortion, however, the Court, will be asked, to unduly prefer religion over non-religion and to substantially burden non-religious individuals! The issue should be decided on purely scientific and medical grounds,
with the protection of the rights of the non-religious in mind.. Religious beliefs should not be allowed to enter into the Court's decision. Allowing abortions according to scientific and medical facts in no way tramples on anyone's right to free exercise of their religion, as no person is required to abort a pregnancy. What those who would choose religious over secular answers would do, is prevent others from exercising their rights. Freedom
from religion is an inalienable right under our U.S. Constitution. We are born with that right.