Hey there, I struggle with this basic market sizing reasoning:

Assume that, on average, a Chinese woman has 2 babies in her lifetime (pregnancy: 18 months = 1½ years). If we assume a life expectancy of 75 years, then we have on average **2%** of all female population pregnant at a given time (1.5 years/75years).

What I get: 2 percent of her lifeTIME spends a Chinese women being pregnant on avg.

What I don't get: why can we equal this 2% of an average lifetime with 2 percent of the female population pregnant at a given time.

I really don't get the reasoning behind it. Are there implicit assumptions not stated here? I would be very happy for some easy, clear (and alternative) examples to get the reasoning.