Proestrus is defined as:

Proestrus is defined as:

• A. The period immediately before estrus, characterized by development of the endometrium and ovarian follicles
• B. The phase during estrus
• C. The luteal phase with corpora lutea function
• D. The period of implantation

Answer: (A)

Proestrus is the lead-up to ovulation, a preparatory stage where the ovaries are developing follicles and estrogen production is increasing. This hormonal rise stimulates the uterine lining to thicken and become ready for possible implantation, so the reproductive tract is primed for mating and potential pregnancy. It ends as estrus begins, when the female becomes receptive and ovulation is approaching. The other options don’t fit because the phase during estrus is the period of sexual receptivity, not the preparatory follicular development seen in proestrus. The luteal phase occurs after ovulation and involves the corpus luteum and progesterone, not the follicular development that characterizes proestrus. Implantation happens after fertilization, within the uterus, not during proestrus.

Proestrus is the lead-up to ovulation, a preparatory stage where the ovaries are developing follicles and estrogen production is increasing. This hormonal rise stimulates the uterine lining to thicken and become ready for possible implantation, so the reproductive tract is primed for mating and potential pregnancy. It ends as estrus begins, when the female becomes receptive and ovulation is approaching.

The other options don’t fit because the phase during estrus is the period of sexual receptivity, not the preparatory follicular development seen in proestrus. The luteal phase occurs after ovulation and involves the corpus luteum and progesterone, not the follicular development that characterizes proestrus. Implantation happens after fertilization, within the uterus, not during proestrus.