Guinea pig sows from
standard-sized pet strains are best mated for the first time
when they are between 500 - 600 grams of weight (550g
average), minimum 450 grams. Sows will generally be
between 2.5 to 3 months old when they attain the correct
weight. Most sources indicate attained body weight as the
primary consideration while remaining within the target age
range.
Recommended mating weights can vary between
guinea pig strains. Some producers of laboratory strains use
350g - 500g as their guideline, but 350g is smaller than my
pet strains would reasonably go.
Sows should be successfully mated no later
than 3 - 4 months of age, with the aim of having the sow
deliver her first litter between 4.5 - 6 months of age. The
earlier part of this age range is preferred, provided the
sow's size supports it. The goal in the timing of this first
delivery is for the sow to be large enough to carry the litter
safely, yet achieve the first birth while the sow is as young
as possible.
Young age in the sow at first delivery is a
crucial point because, between 6 and 9 months of age, the
pelvis of a sow that has not given birth will calcify and
become permanently stuck a closed configuration, a condition
called
firm fusion of the symphysis pubis ("fused
pelvis"). If a sow becomes pregnant and a fused pelvis is
present, the pelvis will not spread into an open shape for
delivery, making vaginal delivery of babies impossible and
necessitating cesarean delivery to save the sow's life.
(Sows that become pregnant for the first
time significantly later than this should have an
exotic-appropriate veterinary surgeon arranged in advance for
a possible cesarean delivery.)
Additionally, ensuring that breeding sows
are mated before 6 months of age can help prevent them from
becoming too fat.
I read in one source that a single
successful vaginal delivery should confer lifetime protection
from a fused pelvis, but I can't remember which source this
was.
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Sources:
Wagner, Joseph E.; Manning, Patrick J (1976). The Biology of the Guinea Pig.
Academic Press. ISBN 978-0-12-730050-4.
Lynn C. Anderson, Glen Otto, Kathleen R. Pritchett-Corning,
Mark T. Whary (2016). Laboratory Animal Medicine, Third
Edition.
Academic Press. ISBN 978-0-12-409527-4.
This article also include impressions and information
directly from the author's experience.