Pregnancy can cause skin to darken in the face, nipples, and even in a line from the belly button to the pubic region. This darkening can sometimes be referred to as a "glow", similar to someone with a tan might be considered to glow.
> In the second trimester (13-28 weeks) a woman begins to look noticeably pregnant and the enlarged uterus is easy to feel. The nipples get bigger and darker, skin may darken, and some women may feel flushed and warm.
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