Hello, I'm an expert in pediatric dermatology, and I can explain how Mongolian spots occur.

Mongolian spots are a common type of birthmark that are more prevalent in certain ethnic groups, particularly those with ancestry from East Asia, but they can appear in individuals of any background. They are characterized by flat, blue-gray patches of skin, usually found on the buttocks or the back.

Mongolian spots are thought to be caused by an excess of melanin, the pigment that gives color to our skin, hair, and eyes. The overproduction of melanin is believed to be due to a genetic factor. During fetal development, an overproduction of melanocytes (cells that produce melanin) can lead to the formation of these spots. They are not harmful and usually fade as the child grows older.

Here's the translation in Chinese:

你好，我是儿科皮肤科的专家，我可以解释蒙古斑是如何产生的。

蒙古斑是一种常见的胎记，特别是在某些种族群体中更为普遍，尤其是那些具有东亚血统的人，但它们也可以出现在任何背景的个体中。它们的特征是平坦的蓝灰色皮肤斑块，通常位于臀部或背部。

人们认为蒙古斑是由过量的黑色素引起的，黑色素是我们皮肤、头发和眼睛颜色的色素。过量产生黑色素被认为是由于遗传因素。在胎儿发育期间，黑色素细胞（产生黑色素的细胞）的过量产生可以导致这些斑点的形成。它们是无害的，并且随着孩子年龄的增长通常会消退。

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Mongolian spots are very common in any part of the body of dark-skinned babies. They are flat, gray-blue in color (almost looking like a bruise), and can be small or large. They are caused by some pigment that didn't make it to the top layer when baby's skin was being formed. ... Read more about newborn's skin and rashes.read more >>