Most vulnerable period for teratogenicity during fetal development is

The preimplantation period is the 2 weeks from feilisation to implantation and is known as the "all or none" period. As the zygote undergoes cleavage, an insult damaging a large number of cells typically causes embryonic death.However if few cells are injured compensation may be possible, with normal development. The embryonic period is from the 2nd -8th week post feilisation, encompasses organogenesis and is thus the most critical period with regard to structural malformations. The fetal period, which is beyound 8 weeks, is characterised by continued maturition and functional development and during this time, ceain organs remain vulnerable. Reference: William's Textbook of Gynaecology 4th edition page 242