In the study on congenital heart defects, the use of specific antidepressants (selective serotonin reuptake inhibitors (SSRIs)) during the 1st trimester of pregnancy was associated with a small, but statistically significant higher risk of some congenital heart malformations. Use of SSRI's in other trimesters of pregnancy was not association with these malformations. In this example, the 1st trimester of pregnancy is an example of which of the following? Choose the one best answer.

	A.	Biomarker of exposure
	B.	Critical window
	C.	Latency period
	D.	Time-varying exposure