The reason for the hidden pain in the stomach on the 9th day of IVF transplantation may be caused by the implantation of fertilized egg, preeclampsia, ectopic pregnancy and other reasons. 1. Fertilized egg implantation: the process of fertilized egg implantation makes the estrogen and progesterone levels in the body change, the hormonal changes stimulate the pelvic and uterine cavities, and in a state of congestion, you may have a vague pain in the stomach. 2. Pre-eclampsia: Rejection reaction or insufficient progesterone in the body after in vitro fertilization may cause pre-eclampsia, which stimulates the uterus to contract and may cause vague pain in the stomach. 3. Ectopic pregnancy: although the embryo is planted in the uterine cavity after the transplantation, the fertilized egg will wander to the fallopian tube to have an ectopic pregnancy, and there will be hidden pain in the stomach at the beginning. There are more reasons for the hidden pain in the stomach on the 9th day of IVF transplantation, which lasts for a longer period of time, so you should go to the hospital for examination and diagnosis.