During which stage of pregnancy are syphilis and rubella considered the most dangerous?

Syphilis and rubella are particularly hazardous during the first trimester of pregnancy because this is when organogenesis occurs — the period during which the critical structures and organs of the developing fetus are forming. The first trimester spans from conception to the end of 12 weeks, and exposure to teratogens, such as certain infections, can lead to serious developmental issues.

In the case of rubella, the risk of congenital rubella syndrome is greatly elevated if the mother contracts the virus during this early stage, potentially causing severe birth defects, including heart problems and deafness. Similarly, syphilis can also have devastating effects if the mother is infected during the first trimester, leading to miscarriage, stillbirth, or congenital syphilis, which may result in significant health complications for the newborn.

As pregnancy progresses into the second and third trimesters, while risks still exist, the likelihood of severe congenital effects decreases significantly; is more associated with maternal health and managing the condition rather than the initial formations of vital organs and limbs. Therefore, the first trimester is indeed the most critical period regarding the impacts of these infections on fetal development.