The third trimester is defined as starting, between the beginning of week 28 (27 weeks + 0 days of GA) or beginning of week 29 (28 weeks + 0 days of GA). It lasts until childbirth. Estimation of due date
Due date estimation basically follows two steps:

Determination of which time point is to be used as origin for gestational age, as described in the section above. Adding the estimated gestational age at childbirth to the above time point. Childbirth on average occurs at a gestational age of 280 days (40 weeks), which is therefore often used as a standard estimation for individual pregnancies. However, alternative durations as well as more individualized methods have also been suggested.The American College of Obstetricians and Gynecologists divides full term into three devisions:
Early-term: 37 weeks and 0 days through 38 weeks and 6 days
Full-term: 39 weeks and 0 days through 40 weeks and 6 days
Late-term: 41 weeks and 0 days through 41 weeks and 6 days
Post-term: greater than or equal to 42 weeks and 0 daysNaegeles rule is a standard way of calculating the due date for a pregnancy when assuming a gestational age of 280 days at childbirth. The rule estimates the expected date of delivery (EDD) by adding a year, subtracting three months, and adding seven days to the origin of gestational age.
Gradually, portraits of pregnant women began to appear, with a particular fashion for "pregnancy portraits" in elite portraiture of the years around 1600. Pregnancy, and especially pregnancy of unmarried women, is also an important motif in literature. Notable examples include Thomas Hardys 1891 novel Tess of the dUrbervilles and Goethes 1808 play Faust. See also
Couvade syndrome
Cryptic pregnancy
False pregnancy
Simulated pregnancy

References
Further reading
External links

Pregnancy at Curlie
Merck Manual Home Health Handbook – further details on the diseases, disorders, etc., which may complicate pregnancy. Pregnancy care – NHS guide to having a baby including preconception, pregnancy, labor, and birth.