The females of all livebearers have a so-called gestation spot.
The pregnancy patch is a dark spot in front of the anal fin, slightly behind and below the swim bladder.
The gestation patch develops when the females become sexually mature.
The stain is distinctly different.
In fish species with a strong skin color, such as swordtails, platies and black mollies, the stain is difficult or impossible to see.
Guppy female left male right Source: Image on Wikimedia Commons License: Public domain Author: Rchampagne
It is usually clearly visible in guppies.
Swordtail Photo: Heiko Kühn
Sometimes you can also tell from the location of the gestation spot whether a female is carrying young. The eggs shine through the skin at the site of the gestation patch. If you look closely, you can see the individual eggs in good light, transmitted light.
In the case of heavily pregnant females, you may even see the young fish shimmering through their stomachs.
The black spot on the swordtail below appears to be more of a pigmentation mark in the swordtail’s skin, rather than a pregnancy mark. In dark red animals, the pregnancy spot is also more reddish.