What happens at a Baseline Appointment? It consists of a trans-vaginal ultrasound, a consultation with the doctor, and possibly blood-work. They will come up with several numbers. Around here, the uterine lining measurement will be in millimeters, and they will say how many follicles shown in each ovary, and how big they are. For a medicated Frozen Embryo Transfer, they do not want you to ovulate, because they are going to build up and prepare your lining with medication, planning to transfer your embryo(s) when the lining should be just right, around 21 days after you start taking estrogen. (They’ll check the lining about a week before transfer to make sure.) Ovulating would mess up the timing of that process. For a natural FET, they hope to see at baseline that your body is preparing follicles for ovulation, because they plan for your body to build and prepare your lining naturally, in about 2 weeks, depending. They’ll be checking both the ovaries and the uterine lining frequently, because they will time your transfer for soon after you ovulate.