Fetal response to Treponema pallidum occurs at which week of pregnancy?

The risk of fetal infection depends on the degree of maternal spirochetemia (greater in the secondary stage than in the primary or latent stages) and the gestational age of the fetus. Treponemes may cross the placenta at all stages of pregnancy, but fetal involvement is rare before 18 weeks. After 18 weeks, the fetus is able to mount an immunologic response, and tissue damage may result. The earlier in pregnancy the fetus is exposed, the more severe the fetal infection and the greater the risk of premature delivery or stillbih. Ref: Mackay G. (2013). Chapter 43. Sexually Transmitted Diseases & Pelvic Infections. In A.H. DeCherney, L. Nathan, N. Laufer, A.S. Roman (Eds), CURRENT Diagnosis & Treatment: Obstetrics & Gynecology, 11e.