According to this Wall
Street Journal blog
piece:

That’s the question of new, dueling research
articles out in the February issue of the Journal of Marriage and Family, which
try to explain the decline in marital happiness of some new mothers.

I don’t need a research article (never mind two) to
answer that question. When I had my first baby, my “decline in marital
happiness” could be summed up in one word: exhaustion. Chronic sleep
deprivation can undo your sanity, never mind your relationships. But I’m being
facetious. Overall, my marital happiness didn’t decline; it changed—into something
richer, fuller, certainly more challenging, but ultimately soul-expanding. My
marriage changed, but that’s what you can expect with any major life
transformation, which parenthood undoubtedly is.

This line is slightly amusing:

To be sure, many women
stay happy, or may become happier, when they make the transition to motherhood,
especially those who “believe that one of the central purposes of marriage is
childbearing.”

I certainly count myself among the number of women who believes
that marriage ought to have something to do with raising children, but given society’s
drastic redefinition of concepts like marriage and family, one can’t take that
sort of thing for granted any more.

If you believe that the central purpose of marriage is to have fun
with your partner, in a perpetual adolescence, then perhaps having children is
just a drain on your time, energy and resources. Hence, the high divorce rate
and demographic decline in the West.

Although many women stay
happy, and may become happier, when they become mothers, some do not.
Researchers used a survey of 569 wives and found that one of the key drivers of
new mothers’ dissatisfaction was the decline in quality time with spouses

If that’s the case, the problem is not the presence of children,
but the motivation of the spouses to stay connected. New parenthood is
certainly a test of a couple’s love and mutual commitment. I’m showing my
female bias here, but I believe the onus is slightly greater on the new dad to
step up his efforts. (The 24/7 intensity of breast-feeding and principal
childcare of newborns doesn’t leave mums much time or energy for creativity in
the relationship.) On the other hand, a hug, a backrub, or an encouraging word
from your husband can go a long way.

My husband and I have seven children, and we’ve been married
almost 25 years (happier than ever, I might add). That didn’t happen by magic. Marital
satisfaction takes effort; it is difficult, if not impossible to achieve if
either partner is unwilling to contribute to the
other’s happiness. Perhaps it goes beyond the question of the purpose of
marriage to the very purpose of human existence.

I
liked the conclusion of the WSJ post:

Prof. Milkie calls for a
more “flexible and forgiving culture of motherhood” that would enable wives to
choose to devote more time to selves and other relationships without concern
that they were neglecting their children, all in a time in which, she claims,
mothering has become more intense.

Spouses in general need to be more "flexible and forgiving". Perhaps
the secret to marital happiness is finding balance: between your legitimate needs
as a person with a unique spiritual and physical dimension, and the willingness
to sacrifice when necessary (it’s necessary a lot); and balancing the needs of
your spouse and the needs of your children. With plenty of unconditional love
all round.