The interval of time from the vasectomy.
Studies have shown success is related to the timing of the vasectomy which in turn is related to the ability of the testicles to begin producing sperm again in adequate numbers to achieve pregnancy.
Click on “Success Rates” in Menu to see how the interval of the vasectomy affects success.
From The Atlantic
A handful of studies have tried to pinpoint a number of children that maximizes parents’ happiness. One study from the mid-2000s indicated that a second child or a third didn’t make parents happier. “If you want to maximize your subjective well-being, you should stop at one child,” the study’s author told Psychology Today. A more recent study, from Europe, found that two was the magic number; having more children didn’t bring parents more joy.
In the United States, nearly half of adults consider two to be the ideal number of children, according to Gallup polls, with three as the next most popular option, preferred by 26 percent. Two is the favorite across Europe, too.
Ashley Larsen Gibby, a Ph.D. student in sociology and demography at Penn State, notes that these numbers come with some disclaimers. “While a lot of [the] evidence points to two children being optimal, I would be hesitant to make that claim or generalize it past Western populations,” she wrote to me in an email. “Having the ‘normative’ number of children is likely met with more support both socially and institutionally. Therefore, perhaps two is optimal in places where two is considered the norm. However, if the norm changed, I think the answer to your question would change as well.”