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The Wikipedia article on fetal viability gives a table giving fetal viability at each week of gestation:

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So at 34 weeks the viability rate is >98%. But I'm wondering what is the viability of a full-term baby, that is, at 39 to 40 weeks of gestation? The reason I ask this is because I assume that some full-term babies don't have a 100% viability.

Also, does anyone know what viability means exactly? In the article I checked it's defined as

"The ability of a fetus to survive outside the uterus."
Fetal viability (Wikipedia article)

I'm not sure exactly what this means, because it seems to me vague. For example infant mortality rate is not vague, it specifically expresses the number of babies that die within the first year of their life, but the fetal viability measure doesn't mention a fixed time period. For example would a baby who manages to live for 6 months be considered to be viable or have been considered to be viable?

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The concept of fetal viability is really only relevant when discussing premature infants. You are correct that the wikipedia figures are difficult to interpret. They come from this website, which is not itself a peer reviewed source, does not declare the source of these data, and do not define their terms. Unlike vital statistics like infant mortality, fetal viability does not have a standardized definition. One excellent study, often used in clinical decision making, used survival (and neurodevelopmental outcome) at age 18-22 weeks (corrected for gestational age at birth). As an example of the difference in definition, another used survival at discharge.

If you are interested in vital statistics for term infants, though, searching for data on fetal viability will not be particularly helpful. US data were recently reported and analyzed in last July's JAMA Peds. See this article.

Here is a chart that might provide what you're looking for. Recognize that these are descriptive statistics only, and do not address underlying changes in practice and policy over the period described. To help you interpret this chart, stillbirth and neonatal death are standardized terms. A stillbirth is fetal death prior to birth (no signs of life on delivery), neonatal death is death after birth up to 27 days.

enter image description here

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  • Looking at that chart your provided, any idea why the number and percent of stillbirths are higher for 32-33 weeks than for 28-31 weeks?
    – Zebrafish
    Jan 30, 2019 at 17:46
  • @Zebrafish I'm not sure what you mean. The number of stillbirths in the 32-33 week category is lower than the number in the 28-31 week category for each year. The rate (the value inside the parentheses) is per 1000, not per 100 (percent). If you're asking why the rate of stillbirth is higher in 32-33 than 28-31, you could probably draw some conclusions by comparing the rates of neonatal deaths in those same categories.
    – De Novo
    Jan 30, 2019 at 18:09
  • @DeNovo This is far from my area of expertise, but I suspect this is from treatment to delay birth when labor begins during those weeks. The numbers of both deaths and stillbirths are low in that range, and I suspect the denominator is also small (i.e., there are few live births) which gives a relatively large rate of stillbirth.
    – Bryan Krause ♦
    Jan 30, 2019 at 18:39
  • @BryanKrause interventions to delay birth occur in both of these preterm categories, so births at 28-31 would be subject to a similar effect (as 32-33). The point I was trying to lead to is that the apparent pattern is reversed when looking at neonatal deaths. 28-31 has lower stillbirth, higher neonatal death. 32-33 has higher stillbirth, lower neonatal death.
    – De Novo
    Jan 30, 2019 at 19:26
  • @BryanKrause I'm not aware of a study investigating this with any rigor, but it would be reasonable to test a hypothesis that some fetal interventions in the 28-31 group may improve short term outcomes (survival to delivery), but not result in survival beyond the neonatal period.
    – De Novo
    Jan 30, 2019 at 19:30

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