Liz - 5 mins thought before posting would probably help us see a clearer question.
If I understand you
(a) a manxborn female is in Wavertree workhouse in 1901 age 13
(b) could she pregnant - possible but unlikely, age of puberty was often later especially for those on poor diet
(c) could she return to the Island with her child - yes but she may have problem with employment unles her family could look after the child (assuming said hypothetical child survives
(d) could she get pregnant again age 21 - almost certainly yes - and statistically more likely as having had one illegitimate baby she is less attractive as a marriage partner and thus more likely to take risks to find a partner