Answer: Progesterone.
Step-by-step explanation:
In a mature female if fertilization occurs, the egg gets attached to the endometrium layer of uterus.
The pituitary gland secretes luteinzing hormone and follicle stimulating hormone.that stimulates ovary to produce estrogen and progesterone.
Estrogen is responsible for thickening of the endometrial lining and progesterone keeps endometrium intact so that the fertilized egg can be successfully implanted.