Which stage in human fertilisation/conception/pregnancy period does susceptibility to environmental factors that may contribute/cause defects start?

Before conception: The woman is born with all the eggs she will ever have and they mature over her lifetime until either released or lost.All exposure to chemicals/radiation/illnesses during a woman's life impact the egg quality.Once released to conception, the environmental influences on outcome are continuous.Some are time specific (thalidomide-5-7 wks) some are continuous (alcohol).