One of my own stories has a sub-species of the OG mermaids (all female) which are (but rarely) male and they can get pregnant in tail form, by a similar method, regardless of gender (males can't change back to their legs during the pregnancy tho). I don't know why, but I imagined that would be the case here as well, but too bad it wasn't. Would have been an extra gimmick.
One of my own stories has a sub-species of the OG mermaids (all female) which are (but rarely) male and they can get pregnant in tail form, by a similar method, regardless of gender (males can't change back to their legs during the pregnancy tho). I don't know why, but I imagined that would be the case here as well, but too bad it wasn't. Would have been an extra gimmick.