An identifier which indicates the number of babies born and their rank (i.e. birth order).

National Codes:

1/1One baby
1/2First of two babies (twin 1)
2/2Second of two babies (twin 2) 
1/3First of three babies (triplet 1)
2/3Second of three babies (triplet 2)
3/3Third of three babies triplet 3)
1/4First of four babies (quadruplet 1)
2/4Second of four babies (quadruplet 2)
3/4Third of four babies (quadruplet 3)
4/4Fourth of four babies (quadruplet 4)
1/5First of five babies (quintuplet 1)
2/5Second of five babies (quintuplet 2)
3/5Third of five babies (quintuplet 3)
4/5Fourth of five babies (quintuplet 4)
5/5Fifth of five babies (quintuplet 5)
1/6First of six babies (sextuplet 1)
2/6Second of six babies (sextuplet 2)
3/6Third of six babies (sextuplet 3)
4/6Fourth of six babies (sextuplet 4)
5/6Fifth of six babies (sextuplet 5)
6/6Sixth of six babies (sextuplet 6)
1/7First of seven babies (septuplet 1)
2/7Second of seven babies (septuplet 2)
3/7Third of seven babies (septuplet 3)
4/7Fourth of seven babies (septuplet 4)
5/7Fifth of seven babies (septuplet 5)
6/7Sixth of seven babies (septuplet 6)
7/7Seventh of seven babies (septuplet 7)
1/8First of eight babies (octuplet 1)
2/8Second of eight babies (octuplet 2)
3/8Third of eight babies (octuplet 3)
4/8Fourth of eight babies (octuplet 4)
5/8Fifth of eight babies (octuplet 5)
6/8Sixth of eight babies (octuplet 6)
7/8Seventh of eight babies (octuplet 7)
8/8Eighth of eight babies (octuplet 8)
1/9First of nine babies (nontuplet 1)
2/9Second of nine babies (nontuplet 2)
3/9Third of nine babies (nontuplet 3)
4/9Fourth of nine babies (nontuplet 4)
5/9Fifth of nine babies (nontuplet 5)
6/9Sixth of nine babies (nontuplet 6)
7/9Seventh of nine babies (nontuplet 7)
8/9Eighth of nine babies (nontuplet 8)
9/9Ninth of nine babies (nontuplet 9)

This attribute is also known by these names: