3. “The age of the mother is determined by the easily
verified date when the pregnancy ends, not by the
estimated date of conception.
Consequently, the statistics do not include
pregnancies that began in women aged 19 if they
did not end until on or after the woman's 20th
birthday.
Similarly, statistics on the mother's marital status
are determined by whether she is married at the
end of the pregnancy, not at the time of
conception.”
http://en.wikipedia.org/wiki/Teenage_pregnancy#Definition