I donโt know whether anyone has done testing with pregnancies yet but Iโve been doing that with my current family. The main sim, Isolde, created a set of fertility jewellery. She put it on after she got pregnant the first time - didnโt change anything about the pregnancy. The second time, she had the charged jewellery on before getting pregnant and got pregnant with twins. However, because she didnโt recharge the jewellery, when she went to have the baby at the hospital, she only had one of the twins. The third time, she put on half the jewellery at the start of the pregnancy and became pregnant with twins. Halfway through the pregnancy, she swapped jewellery items so she was always wearing charged items, and it resulted in both twins being born. So it seems that your sim has to have the jewellery charged all the time (or at the very least, at the start and end of the pregnancy).