The effects of birth interval on the birthweights of consecutive, same-sex term siblings.

OBJECTIVE

To investigate the effect of birth interval on the relationship between the birthweights of successive siblings.

METHODS

The records of women who had delivered two consecutive, same-sex, live, singletons babies (1983 through 1997) in a private hospital were analysed. The intervals between the birth dates of the siblings were calculated. The first of the siblings was coded Set A and the second, Set B. The ratios of the birthweights (Set B/Set A) were calculated. Comparison of findings was made between various ranges of birth interval.

RESULTS

The birth interval band 24-29 months was associated with the highest mean birthweight for Set B babies and the highest birthweight ratio (p=0.016). Set A babies weighing less than 3000 g were most likely to be outweighed by their Set B siblings (p=0.000001).

CONCLUSION

Birth intervals of 24 to 29 months and small size of Set A babies were associated with most significant advantages in birthweight for Set B siblings.