In order for pregnancy to occur when sperm enters the female reproductive system, what process has to occur first?

A pregnancy starts with fertilization, when a woman's egg joins with a mans sperm.

I hope this helps :)

The answer is “ovulation”. In order for pregnancy to occur, the female’s ovaries must release one or more fertile eggs. A pregnancy requires both eggs and sperm, and the two must unite inside the female reproductive system in order to produce offspring.

Explanation:

In humans, ovulation usually occurs once per month, midway between periods, and usually only one egg is released. Every once in a while two or three eggs are released together, which results in fraternal twins or triplets. Pregnancy can only occur if sex happens close to the time of ovulation, because human eggs remain fertile for only about 24 hours after they are released.

But it is important to note that ovulation does not have to occur before the entry of sperm. In humans, sperm can remain fertile for up to 5 days inside the female reproductive system. If she ovulates at any time during those 5 days, pregnancy can occur.