OCR GCSE Biology 2025 – 400 Free Practice Questions to Pass the Exam

The hormone that causes the lining of the uterus to thicken is oestrogen. During the menstrual cycle, oestrogen is primarily produced by the developing follicles in the ovaries and plays a crucial role in the preparation of the uterus for potential implantation of a fertilized egg.

As oestrogen levels rise in the first half of the menstrual cycle, it stimulates the endometrium, or uterine lining, to grow and become thicker. This thickening provides a nourishing environment that is ideal for a fertilized egg to implant and develop.

In contrast, other hormones listed have different functions. For instance, FSH (follicle-stimulating hormone) is mainly responsible for stimulating the growth of ovarian follicles but does not directly cause the thickening of the uterine lining. LH (luteinizing hormone) triggers ovulation and is more involved in the later stages of the cycle. Progesterone, while important for maintaining the uterine lining after ovulation, works primarily to stabilize and prepare the endometrium rather than initiate its initial thickening.

Thus, oestrogen is the key hormone responsible for the increase in uterine thickness leading up to the possibility of pregnancy.