The gestational age of an unborn child is its age since conception. The life of a prenatal human person begins at conception. Viability is reached when the prenatal has developed to a sufficient extent, in the womb, in order to survive outside the womb. However, viability is not a single sharp line, but rather an increasing chance of survival after being born. Thus, viability is often expressed as a percentage of infant who are expected to survive, if born at that particular gestational age.

I reviewed a couple of recent medical articles on the topic. One study:
Early-Childhood Neurodevelopmental Outcomes Are
Not Improving for Infants Born at < 25 Weeks’ Gestational Age
looked at survival rates over two periods of time (two ‘epochs’) for neonatals from two different hospitals specializing in neonatal intensive care. The conclusions were as follows:

Estimated Gestational Age (EGA) and survival rate

“In epoch 1 [1999-2001],
survival according to EGA was as follows:

22 weeks’ or less EGA,
12 of 292 (4.1%) infants;

23 weeks’ EGA,
101 of 395 (25.6%) infants;

and 24 weeks’ EGA,
345 of 606 (56.9%) infants.

In epoch 2 [2002-2004],
survival according to EGA was as follows:

22 weeks’ or less EGA,
13 of 322 (4.0%) infants;

23 weeks’ EGA,
99 of 441 (22.5%) infants;

and 24 weeks’ EGA
338 of 632 (53.5%) infants.”

So the above study found that at 23 weeks or less, the infants had a low chance of surviving premature delivery. But at 24 weeks, the survival rate rose above 50%.

Another study examined viability at a single regional neonatal unit at a tertiary care hospital:
The Limit of Viability: A Single Regional Unit’s Experience
That study found significantly higher rates of survival for younger neonatals (data from Table 1)

For epoch 1 (January 1998 to June 2003), survival rates were:

22 weeks: 20%
23 weeks: 62%
24 weeks: 77%
25 weeks: 77%
26 weeks: 93%

For epoch 2 (July 2003 to December 2008), survival rates were:

22 weeks: 40%
23 weeks: 63%
24 weeks: 81%
25 weeks: 89%
26 weeks: 95%

By 26 weeks, survival rates are well over 90%. At 25 weeks, the survival rate is still relatively high for a premature birth. But notice what happened at 22 weeks. From 1998-2003, the survival rate was about 20%. But this particular hospital was able to double the survival rate at that gestational age to 40%. Of 50 births at about 22 weeks, 20 neonatals survived (second epoch).

The normal delivery window (90% of all births) is from 36 to 40 weeks. So even 26 weeks is very premature. Pregnancy is often divided into three trimesters of 13 weeks each. The prenatal is clearly viable in the entire third trimester. Advances in neonatal care and practice are pushing viability into the late second trimester.

Recent legislation to greatly restrict abortion after 20 weeks has proposed that gestational age as the beginning of the sensation of pain. Of course, the reason that abortion is gravely immoral is not the pain it causes, but that an innocent life is deliberately extinguished. But now that survival rates for neonatals born before 24 weeks are increasing, an argument can be made to further support restrictions after 20 weeks — that the prenatal is either viable or is close to the point of viability.

by
Ronald L. Conte Jr.
Roman Catholic theologian and
translator of the Catholic Public Domain Version of the Bible.