The feminine form is created by replacing -o with -a
 

1st (first)	primeiro (m)
2nd (second)	segundo (m)
3rd (third)	terceiro (m)
4th (fourth)	quarto (m)
5th (fifth)	quinto (m)
6th (sixth)	sexto (m)
7th (seventh)	sétimo (m)
8th (eighth)	oitavo (m)
9th (ninth)	nono (m)
10th (tenth)	décimo (m)
11th (eleventh)	décimo primeiro (m)
12th (twelfth)	décimo segundo (m)
20th (twentieth)	vigésimo (m)
21st (twenty first)	vigésimo primeiro (m)
30th (thirtith)	trigésimo (m)
31st (thirty first)	trigésimo primeiro (m)