I haven't read anything specificly addressing your question, yet I think that the actual conditions (constrained physical and recreational activities, fatigue, medical problems, and perhaps several symptoms, bodily changes and body image problems) related to pregnancy, birth and post partum period and gender roles which usually assign more responsibility of the baby, which may also have constraining effects for the mothers' physical and psychological well-being. Nursing literature about pregnancy, birth and post partum period may help you find detailed discussions about these issues, I think.
Antonio, I think this is related to child`s autonomy, the supports that a mother (or women) can give to children. I had published a paper (Persian) that the results showed mother autonomy supportive had not significant correlation with children`s well-being but this equation was significant for father autonomy supportive. This maybe also be related to negative intimacy or relatedness.
One of most important factors in well-being is related to autonomy and self-determination, hence, when child could not take autonomy support from mother, they maybe effect in decreasing children`s well-being.
You might have a look at the paper called "Does child care involvement increase or decrease distress among dual-earner couples?" on the bottom of page 3 of my profile. Bob
It is well known that in general partnership and in particular parenthood and fatherhood have a significant positive influence on life expectancy of men.
Maybe it is helpful to change the direction of the question: Not decreasing of wellbeing of women through motherhood rather increasing of wellbeing of men through fatherhood.