From embryo to fetus
Pregnancy development is usually divided into an embryonic period and a fetal period. The embryonic period spans roughly the first ten weeks after conception and is when the major organs and body systems first form from the dividing cells. The fetal period that follows, extending to around forty weeks of gestation, is mainly a time of growth and maturation of structures that already exist, rather than the creation of new organs.
Gestational age is conventionally counted from the first day of the last menstrual period, which is about two weeks before conception, so dating terminology can differ between sources.
Why the timing of exposures matters
Each organ has a window during the embryonic period when it is forming and is most sensitive to disruption, so the effect of a teratogen, an agent that can cause malformation, depends heavily on when exposure occurs. Exposure during a critical window can affect the organ developing at that time, while the same exposure later may have little structural effect.
This is general educational information about development, not medical advice; concerns about exposures during pregnancy should be discussed promptly with a qualified clinician.