The role of uterine capacity in determining birth weight
and size has been demonstrated in a number of crossbreeding
experiments between large and small domestic
animal species [1–4]. Embryo transfer experiments between
large (Suffolk) and small (Cheviot) sheep breeds
have offered a further refinement by excluding direct
effects maternal genotype may have on outcome measures
in the offspring [5–7]. These experiments are potentially
very important in gaining a better understanding of the
factors affecting size at birth, which in turn may have a
profound effect on the health [8–11] and productivity of the
offspring [12–14].
Although maternal breed size effects on birth size seem
predictable from these embryo transfer experiments, the
precise mechanisms regulating fetal growth in a constrained
or a relatively unconstrained uterine environment
remain elusive. Simple maternal uterine size is a possible
factor, especially in late gestation, but follow-up studies on
reciprocal embryo transfer between Cheviot and Suffolk
sheep breeds have shown that fetal size is already affected
by maternal size before 90 d gestation [6], well before
spatial constraint of the maternal uterus could be expected
to have full effect. It is highly likely that mechanisms to
modulate fetal growth in a restricted or less restricted
maternal environment are set early in pregnancy.