Prenatal Development Timeline By jm020090 Jan 1, 2017 Start of the period of the zygote Jan 1, 2017 Start of the first trimester Jan 14, 2017 End of the period of the zygote Jan 21, 2017 Start of the period of the embryo Mar 5, 2017 End of the period of the embryo Mar 12, 2017 Start of the period of the Fetus Apr 9, 2017 Start of the second trimester May 1, 2017 Week of sex detection Jun 18, 2017 Week of Viable birth Jul 23, 2017 Start of the third trimester Oct 15, 2017 End of the period of the fetus Oct 15, 2017 Full term pregnancy