Birth size and age at menarche: a twin perspective.
OBJECTIVE
Do birthweight (BW) and co-twin sex influence the age at menarche in twins?


CONCLUSIONS
BW, but not co-twin sex, was associated with age at menarche in twins. However, BW was not associated with age at menarche after controlling for genetics and shared rearing environment.


BACKGROUND
Nutritional deprivation during critical developmental periods can trigger long-term effects on health. A small size at birth has been associated with early age at menarche in singletons. However, the relative influence of genetics and environmental factors on this association remains unresolved.


METHODS
In total, 2505 twin pairs were included in this cohort study.


METHODS
All participants were recruited from the Danish Twin Register. Data on the age at menarche were collected by questionnaire and combined with data on BW, birth length (BL) and gestational age (GA) from the Danish Medical Birth register. The BW for GA standard deviation score (BW-SDS) was calculated.


RESULTS
BW-SDS [hazard ratio (HR) 0.96; 95% confidence interval (CI): 0.93-0.00], P = 0.04], but not BW, BL or GA (P ≥ 0.15), was positively associated with age at menarche in all twins after adjustment for zygosity and year of birth. However, BW-SDS was not associated with menarcheal age within twin pairs (HR 1.01; 95% CI: 0.91-1.12, P = 0.88). No differences were found in the age at menarche or birth size between twin girls from same sex and twin girls from opposite-sex pregnancies. Heritability of menarcheal age and BW were estimated to be 0.61 (95% CI: 0.38-0.84) and 0.27 (95% CI: 0.18-0.38), respectively. Both BW and menarcheal age were influenced by genetic and environmental factors.


CONCLUSIONS
A limitation of this study is recall bias on the age at menarche. It is also not clear how these results should be extrapolated to the non-twin population.


CONCLUSIONS
lower BW for GA is associated with earlier age at menarche in twin girls. However, the lack of within-pair differences in menarcheal age between even markedly BW-discordant twins indicates that this association is governed by environmental or genetic factors shared by both twins. Thus, within-pair differences in intrauterine nutritional factors leading to discordant growth do not seem to influence timing of menarche.


BACKGROUND
The authors have nothing to declare. Departmental funds were used to support all authors during the study period and preparation.
Department of Growth and Reproduction GR, Copenhagen University Hospital, Rigshospitalet Section 5064, Blegdamsvej 9, Copenhagen Ø DK-2100, Denmark.
Kaspar
K
Sørensen
Anders
A
Juul
Kaare
K
Christensen
Axel
A
Skytthe
Thomas
T
Scheike
Tina
T
Kold Jensen
D000293:Adolescent; D000367:Age Factors; D001724:Birth Weight; D015331:Cohort Studies; D003718:Denmark; D005260:Female; D006801:Humans; D008297:Male; D008572:Menarche; D012737:Sex Factors; D014427:Twins
birthweight; menarche; sex; twins