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.
Halls is a plural of the word hall. Halls may also refer to:

People
Walter Halls (1871–1953), British trade unionist and politician
Ethel May Halls (1882–1967), American actress
Julian Halls (born 1967), British field hockey player
Evelyn Halls (born 1972), Australian fencer
Roxana Halls (born 1974), English artist
Monty Halls (born 1976), British marine biologist and TV presenter
John Halls (born 1982), English footballer, mostly played for Stoke, Brentford and Aldershot, and model
Andy Halls (born 1992), English footballer, has played for Stockport, Macclesfield and Chester
Halls (footballer) (born 1999), Brazilian footballer
Henrique Halls (born 2002), Brazilian footballer

Places
Halls, Georgia, an unincorporated community
Halls, Missouri, an unincorporated community
Halls, Tennessee, a town in West Tennessee
Not to be confused with Halls Crossroads, Tennessee, a suburb of Knoxville sometimes colloquially referred to as "Halls"

Business
Halls (cough drop), a brand of cough drop
Geo. Hall & Sons, a defunct Australian soft drink company
Halls (department store), Kansas City, Missouri, United States