| 1. First trimester 4 weeks or less | No blood supply connecting mother & fetus; Fetal organs have not yet started to form | Minimal |
| 2. First trimester 4-10 weeks | Individual organs form over specific time point during this period e.g. limb buds (arms & legs) develop at week 6 while the heart divides into different cavities at week 7. | Potential for birth defects where the adverse effect links with the timing of target organ being formed |
| 3. First trimester 11-13 weeks | Fetal organs require nutrition to grow and begin to function | Minimal |
| 4. Second trimester 13-26 weeks | Fetal organs require nutrition to grow and begin to function | Minimal |
| 5. Third trimester 27-40 weeks | Fetal organs practice functioning to prepare for delivery but are immature and less efficient at nutrient and medication elimination than the mother. | Potential for Increased medication levels in the fetus which can cause toxicity & in some cases withdrawal effects postpartum |
