Niveoscincus microlepidotus, the southern snow skink, is a biennially reproducing alpine viviparous lizard with an extremely protracted gestation period: embryos are fully developed in autumn, but held over winter so that offspring are born in spring. The obvious benefits for offspring survival of delaying birth until spring have been demonstrated previously. To examine the consequences of deferred parturition for offspring characteristics, we compared neonates obtained in autumn by dissection with neonates born naturally in the spring. Our results demonstrate that deferral of parturition until spring represents a trade-off between key offspring characteristics (spring neonates exhibit lower growth rates, reduced sprint speed after birth, reduced condition and decreased energy reserves) and offspring size [spring neonates are heavier (wet mass) and longer (snout-vent length)]. Furthermore, when females are placed into cold experimental conditions in spring around the time of natural parturition, this species is able to defer parturition for an additional 4 weeks with no significant effect on offspring characteristics. Our results provide further evidence that flexibility in birth date provides a significant advantage to viviparous lizards living in cold climates.