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There is a vertical black line on a womans belly (shes white caucasian). I heard that its a common fact.

Where does this line come from? Does it have a biological function? I heard about a remnant of a survival feature that would help a newborn baby to reach for the breasts. Any clue?

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    This phenomenon is known as the linea nigra. – Greg Hewgill Apr 12 '16 at 22:22
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    @GregHewgill You should expand that into an answer. – Becuzz Apr 13 '16 at 13:52
  • That makes no sense, IMHO. A human newborn is so weak that it can't reach a breast – it must be placed there by mother. And in far ancestors, more similar to apes, dark colour would camouflage nipples in hair, so it would be rather counterproductive as a 'survival feature', even if their children were able to crawl themselves to mom's breast... – CiaPan Apr 14 '16 at 13:11
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    @Stephie OTOH imagine (pre-human) mother is busy, say walking, climbing on trees or collecting plants or worms for food – the contemporary human's child would be unable to hold on her (unlike monkeys, which can catch the mother's fur and stay on her back since first days). And the 'survival feature' of darkening nipples wouldn't help, either. ....anyway this discussion went far off-topic and it is not going to solve my doubt anyway. – CiaPan Apr 14 '16 at 14:49
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    This question has an answer, so it won't be migrated, but asking the biological the reasons behind it are more appropriate on Biology.SE. – anongoodnurse Apr 14 '16 at 16:33
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The line was there before the pregnancy, then known as linea alba ("white line"), a line of fibrous connective tissue running down your central line - if you as the father should happen to have a "sixpack", the linea alba causes the central dividing line. In most cases, it can not be really seen or at least goes unnoticed.

During the pregnancy, the general melanin production is increased due to hormonal changes. Many women get darker nipples, some dark areas in the face ("mask of pregnancy") or are prone to hyperpigmentation / dark spots if exposed to the sun. The darkening of the linea alba area into what then becomes visible as linea nigra is another example.

While darker nipples sometimes are explained as useful in helping the newborn to find the breast and nurse, I don't think the linea nigra has any biological function, like other areas of higher pigmentation as well.

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The vertical dark line on pregnant women's belly, which is termed as pregnancy line is officially known as Linea Nigra. We can even see this line before pregnancy(then called 'Linea Alba', runs from your navel to pubic bone), is usually pale in color, and this line darkens during pregnancy. The pregnancy line actually starts showing up during 5th month of pregnancy, it can even show up early (anytime during second trimester).

As it is a natural part of pregnancy, there are no preventive measures for Linea Nigra. In order to accommodate the growing baby, women's abdominal muscles stretch and slightly separate, thus pregnancy line starts showing up then. This line generally fades away few days/months after delivery. The exact cause of Linea Nigra is still unknown, but it is believed that, hormonal imbalance, and the hormone called 'melanocyte-stimulating hormone'(which is also assumed to cause the darkening around the nipples) is making the line darker during pregnancy.

According to few studies, newborns when placed on mother's abdomen, has the ability to find mother's breast on her own. It is smell, vision and taste that help the newborn to find the breast.

A study named 'Biological mechanisms for homing in on the nipple' states,

When newborn infants are placed in the prone position between their mother's breasts immediately after delivery, they display a consistent sequence of activity, namely, crawling movement, which brings the infants into contact with one of the nipples, and then active sucking, which eventually occurs within about 1 hr after delivery.

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