As she turned 34, on December 19, 1812, Marie Therese seemed overflowing with happiness. On January 27, 1813, Princess Charlotte gave a ball and noticed that d'Angoulême too was in high spirits. In a letter dated February 7 to her friend, Miss Mercer Elphinstone, the English princess made a special note that at her grand fete, d'Angoulême
proved to be a wonderful dance partner. Marie Therese's' doctor Monsieur Lefebvre, knew the reason for the couples ebullience, and as he would explain to Hue that January: 'At this moment, I am tending to a woman who lives above me and who is pregnant for the first time after more than thirteen years of marriage.'
On January 30, Hue wrote to his wife that Dr Lefebvre had given him the miraculous news that the Duc d'Angoulême would be a father in June. The doctor confirmed this in his own handwriting on the same letter. Madame Hue received the note and added a formal sentence on the parer: 'Monsieur Hue and Monsieur Lefebvre designate Madame the Duchesse d'Angoulême, announcing her pregnancy.' On February 15, 1813, Louise de Conde wrote to her father that she was stunned to hear of the pregnancy as she had heard for years that, while Marie Therese had been in the Temple Prison, the Jacobin guards had bragged about destroying her fertility with a combination of drugs. Marie Therese's joy was to be short lived. Quite a few months into the pregnancy, she suffered a miscarriage and that summer left for Bath to recuperate.

I just realized that I didn't answer your question. Sorry about that!
I think by the time she got pregnant, she was at a place ( I can't remember where) where it would have been relatively safe for the baby. The timing was also about the time that 90% of the people started to almost deify her. Instead of the "scum" people said she was just a few years prior, she was now being called " the Miracle Child".