The feminine form is created by replacing the ending -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)