Maybe this is common knowledge to people that are already parents, but I was shocked to find out that pregnancies are counted from the end of the woman's last period. That means being "1 week pregnant" (not that anyone says that) is not pregnant. Conception generally happens 14 days (2 weeks) after the last period. Thus, by the time you find out you are pregnant, you are already 4 weeks in, but the embryo is only 2 weeks old.
So, full-term is basically a 38 week old fetus. Which makes the premature babies that survive after only 25 weeks gestation even more mind-boggling because the fetus is just 23 weeks old.