The purpose of this study is to
ascertain a physiological relationship between prior lactation and the
probability of subsequent multiple pregnancies in humans, thereby lending
support to the role of insulin-like growth factor in both phenomena. Data were
collected in the present study to determine if there is a direct relationship
between the extent of lactation in prior pregnancies and the chance of
twin/triplet gestations subsequently. Positive results of such a study could
support the proposed biochemical significance of insulin-like growth factor
elevation in promoting multi-fetal pregnancies.
Cows with an enhanced tendency to twin
have elevated levels of serum Insulin-like
Growth Factor (IGF).
IGF is a protein component of milk; bovine milk has a lower level of IGF
than human milk, in general.
Recent reports support the view that twinning in humans is also related in some
way with IGF. Somatotropin-stimulated IGF
augmentation often induces ovarian hyperstimulation in subfertile women
undergoing in vitro fertilization. On the other hand,
one small study found no significant difference in plasma follicle stimulating
hormone (FSH) between mothers of Dizygotic (DZ) twins and mothers of singletons
at any stage of the menstrual cycle.
Insulin-like growth factor, a 70-unit polypeptide,
is a major mediator of FSH action on the
ovary.
IGF increases the sensitivity and responsiveness of ovaries to FSH. Variations
in either FSH or IGF can modify the Spontaneous
Twinning Rate (STR).
The mitogenicly powerful synergistic combination of IGF and FSH enhances
ovarian steroidogenesis, increases primordial follicle recruitment, and reduces
ovum apoptosis, thus increasing the chance for double ovulation. In
humans, conceptions that occur while the mother is breast-feeding result in
twin gestation rates (both monozygotic and dizygotic) nine times higher than in
the general gravid population. Also, women who have ever lactated have a higher
mean plasma IGF level than those who never lactated.
Human grand-multiparas (parity>4)
more often breast-feed than women
with fewer pregnancies.
Also, elderly (age>40) grand-multiparas retain their fertility longer and
conceive twins more often than
do women of low parity.
Thus, the current potential for twinning may be a function of lactation in
prior pregnancies. By its cumulative effect, prolonged lactation might extend
the period of enhanced availability of IGF.
No comments:
Post a Comment