At least, 75% of all pregnant mothers experienced this brownish

At least, 75% of all pregnant mothers experienced this brownish or dark line on their abdomen either by the end of they honeymoon trimester or at exactly three

quarter of they pregnancy.many of them do worry about how this brownish line developed and appear visible on they bellow or abdomen.

This vertical line is called Linea Nigra.

Linea nigra is a brownish or dark vertical line that runs from the abdomen from the pubis to the umbilicus. It is caused by the increase in melanocyte-stimulating hormone which is produced by the placenta.

This hormone also cause melasma and darkened nipple.
So it is nothing to worry about,the line will disappear on it own after delivery as the hormone "Estrogen" decrease a few weeks after you've giving birth to your baby.
The other horizontal line is called CS line. Remember to always see a gynaecologist or go for anti natal during pregnancy.

there you complain to your doctor your problems.