Final answer:
The measuring tool for the number of pregnancies per 100 women per year using a birth control method is the failure rate, which differs with the type of contraception and usage consistency.
Step-by-step explanation:
The measuring tool that refers to the number of pregnancies that occur per year for every 100 women using a particular method of birth control is called the failure rate. The failure rate is the percentage of women who become pregnant using a given method of birth control during the first year of use. This rate can vary significantly between different types of contraception, such as barrier methods, hormonal methods, and intrauterine devices. Moreover, there is a notable difference between failure rates with perfect use and those with typical use, with the latter almost always being higher.