"science of midwifery, the department of medicine which deals with the treatment and care of women during pregnancy and childbirth," 1819, from obstetric (adj.); also see -ics.

"one skilled in obstetrics," 1793, from Latin obstetricia "midwifery" (from obstetricus; see obstetric) on model of physician.

"the practice of obstetrics," late 15c., mede-wifri, "the craft or service of a midwife;" a hybrid from midwife + -ery.

1821, "period of three months," from French trimestre (early 17c.), from Latin trimestris "of three months," from tri- "three" (see tri-) + mensis "month" (see moon (n.)). Specific obstetrics sense is attested from 1900. Related: Trimestrial.