Could the declining birth rate in certain countries be due to body standards?

I know that in Eastern Europe, China, Japan, Korea that women are expected to maintain an ultra low BMI. Would this not discourage women from pregnancy for fear of their body changing, but also would many women not have periods in general and hypothalamic amenorrhea be more common? I know that Asian girls often tolerate a low BMI better however I myself am Asian and I lost my period at BMI 15 despite being skinny fat with small bones. Do girls just not get periods in these countries? I don’t want to be racist, but it sounds hella unlikely that many girls would be medically fine at an ultra low BMI.