Prenatal Development Timeline By FACSHahn Jan 1, 2017 Start of First Trimester Period: Jan 1, 2017 to Jan 14, 2017 Period of the Zygote Period: Jan 15, 2017 to Feb 25, 2017 Period of the Embryo Period: Feb 26, 2017 to Oct 1, 2017 Period of the Fetus Mar 26, 2017 Start of Second Trimester May 1, 2017 Week of sex detection- Week 18 Jun 11, 2017 Week of viable birth- 24 weeks Jul 2, 2017 Start of Third Trimester Oct 1, 2017 Full term pregnancy- 40 weeks