What is the 'pregnancy line'?

More than nine in 10 pregnant women develop a line down their abdomen. (Getty Images)

Pregnant women have come to expect cravings, morning sickness and an aching back.

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The dark line that appears down their abdomen, however, may come as a surprise.

Officially called the linea nigra - Latin for “black line” - more than nine in 10 expectant mothers are said to develop the pigmentation.

What is the ‘pregnancy line’?

Despite its name, the “pregnancy line” is often there even when a woman is not expecting, albeit much fainter.

Running from the navel to the pubic bone, the 0.6-to-1.2cm (0.25-to-0.5 inch) wide mark is referred to as the “linea alba” - white line - in non-pregnant women.

Around five months into pregnancy, the line starts to darken to a brownish colour.

Some find the width and “intensity” of colour increases as the pregnancy progresses, but is consistent down the line at any one time.

It may be accompanied by “ligamentum teres sign”, when the belly button shifts to the right.

The pregnancy line is natural and nothing to be alarmed by.

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A 2005 study by University College Hospital in Ibadan, Nigeria, found 92% of 1,550 local pregnant women developed linea nigra.

More than one in 10 (16%) also had a visible line when not pregnant.

The pregnancy line is said to be more common in darker skin tones.

Why the linea alba darkens during pregnancy is unknown.

DermNet NZ points the finger at “placental hormones, metabolic factors and immunological factors”.

Oestrogen levels rise during pregnancy to support the development of an unborn baby’s organs and the function of the placenta.

These elevated hormones are thought to increase melanin production, the pigment that makes skin tan.

This same rise in oestrogen may also trigger a pregnant woman’s nipples and genitals to become darker.

Sun exposure may make the 'pregnancy line' darker. (Getty Images)

Can the pregnancy line be prevented or treated?

The American Pregnancy Association claims linea nigra cannot be prevented.

DermNet NZ alleges, however, sun exposure may make the pigmentation darker. It therefore advises pregnant women avoid UV rays.

Some suggest folic acid may help ward off the pregnancy line.

All pregnant women are advised to take a 400mcg supplement of folic acid while trying to conceive and in the first trimester.

Folic acid “significantly reduces the chance of neural tube defects, such as spina bifida”.

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Spina bifida occurs when a baby’s spine and spinal cord do not develop properly in the womb, causing a gap in the spine.

This then damages the nervous system, potentially triggering paralysis of the legs and double incontinence.

When it comes to the pregnancy line, there is no specific treatment.

Pregnant women should be reassured the pigmentation will not affect their health or that of their unborn baby.

The marking usually fades shortly before they give birth, but may reappear in subsequent pregnancies.

Bleaching has been tried to hide the pregnancy line, but produced “unsatisfactory results”.