cbrelandas @azibird stated below, the neural tube starts to form at the 3rd week and completely forms by the 4th week. Valproic acid can cause NTD, affecting their formation in weeks 3-4. +4
overathe first 2 weeks are "all or none". so usually if the fetus is exposed to teratogens here, it will either terminate the pregnancy or the baby will be unaffected. +3
submitted by โcheesetouch(249)
Teratogens are most susceptible to give damage in 3-8th weeks (embryonic period - organogenesis) of pregnancy. FA p 596 (2018); teratogens section