Nesting and Prolactin

January 8, 2011 at 7:35 pm

On Monday I started “nesting.”  A sudden urgency to prepare for the arrival of baby #4 hit me like a ton of bricks.  The first project I tackled:  pulling out all the gender-neutral baby clothes and blankets and washing them (even though they were already clean).  I also threw our stash of cheap washcloths for the home birth into the load as well. Later we got the last few supplies we needed from our home birth supply list.  And then I started cleaning my bathroom (the room I anticipate spending most of my labor in).

32 weeks (Christmas Day)

I couldn’t help wondering, as I busied myself, is this “nesting” thing just a logical consequence of my realizing how little time I have left before my baby arrives, could it simply be that I’m motivated by the New Year and its attendant resolutions, or is there really something within my body chemistry triggering my need to ready our nest? There’s no question that the nesting instinct exists within the animal kingdom, but what about within us? Is the human nesting urge for real? Are we, too, being governed by instinct as we prepare for our babies’ births?  Questions like those always get my blogging juices flowing.

I hoped to find some scientific studies of the nesting instinct among humans, but my search brought up mostly studies among animals.  For instance, one of the first things I found was a really old study suggesting that nest-building in rabbits is triggered by a change in the ratio of estrogen to progesterone.  More specifically, it seemed that a sudden decrease in progesterone sent those bunnies nesting (Source). Then I spent some time looking at graphs of hormone levels among humans at the end of pregnancy to check for any connections. Like this one (snagged from here):
It shows a slight decline in progesterone at the end of pregnancy, but some of the graphs I looked at did not.  Further digging led me to a study from 2009 focused on hormonal triggers for labor among humans. It mentioned a “decline in the progesterone:estriol ratio that occurs late in gestation as part of the normal progression to labor” (Source). So maybe we’re not too different from the rabbits in that regard, but it just didn’t seem to be enough to be fully responsible for human nesting behavior, especially since many women start nesting far earlier in pregnancy. It also wouldn’t explain why some fathers exhibit nesting behavior. When I posted on my facebook page, “Hello, nesting urge,” more than one commenter said, “My hubby is the nester!”

Another graph I found (here) led me to another possible explanation: prolactin. Prolactin is a hormone most well known for its role in breastfeeding and milk production, but it plays other important roles as well. Prolactin levels rise steeply during the second half of pregnancy. And the hormonal shifts occurring within a pregnant woman can also impact the people regularly within her vicinity. Studies of males cohabitating with pregnant women may help explain why some fathers are “nesters”:

Over the course of a woman’s pregnancy, the man’s prolactin levels gradually rise. In addition, men exposed to pregnant women and new babies experience a drop in testosterone after birth.

There is little doubt that hormonal changes during pregnancy and lactation are more pronounced in women than men. . . . The point is: even in animals with low levels of joint caretaking, both sexes can be primed to care. Neither birth, nor even the prospect of giving birth, are prerequisites for nurturing. Virgin females, or males, can be primed to nurture merely by prolonged exposure to infants or pregnant mothers. (Sarah B. Hrdy, “Evolutionary Context of Human Development: The Cooperative Breeding Model,” Attachment and Bonding: A New Synthesis, 15)

One of the effects of prolactin is decreased sexual drive. It definitely makes sense in the grand design for the men in pregnant women’s lives to be primed for a transfer of their attention from sexual activity to nurturing behaviors.

In his book, Birth and Breastfeeding, Michel Odent connects prolactin directly to nesting behavior:

[Prolactin] does not only act on the breast; it is the basis for nest building in animals. . . . Studies of breastfeeding mothers and of the symptoms suffered by men and women with prolactin-secreting tumors have increased our knowledge of the behavioral effects of this hormone on humans. One such effect is the reduction of libido or sexual interest. In addition, prolactin tends to engender subordinate and submissive states of mind. . . . These behavioral effects are easily explained in terms of the survival of the species. . . . The mother’s subordinate state increases her adaptability to the needs of the baby. (p. 118-119)

Dario Nardi, Ph.D., professor at University of California at Los Angeles, explains, “High prolactin inhibits testosterone, depresses sensation and alertness, and promotes mild inertia and fatigue. This mild inertia has been described as a non-anxious meditative state conducive to nesting behavior. . . . Prolactin also rises during winter months. Low prolactin causes restlessness and lack of interest in nesting behaviors” (Source).

Anthropologists, Carsten Schradin and Gustl Anzenberger, describe prolactin as “the hormone of parenthood,” as it both primes for and increases paternal and maternal behavior in animals and humans. But I find their discussion of the complexity of the factors influencing these behaviors especially helpful:

Although the indications of a relationship between [prolactin] and paternal [or maternal] care are overwhelming, it is clear that [prolactin] cannot be the only factor. . . . Life-sustaining processes generally depend on numerous factors as a kind of insurance, so that they will continue to operate, albeit at a reduced level, after the breakdown of one factor. . . . Other possible factors influencing the occurrence of paternal [or maternal] care in addition to [prolactin] are neonatal hormone levels, changes in steroid hormone concentrations, experience of the male [or female] (social and sexual experience, previous rearing of young), and stimuli emanating from [offspring]. (Source)

So I guess I’m back where I started, in the end, albeit with a slightly better understanding of how prolactin levels impact behavior. Is it likely that hormonal shifts prompt us (and those who love us and live with us) to show heightened interest in baby preparations? I’d say so. Would we probably show heightened interest in those things whether our hormones told us to or not? The answer is probably, “Yes, most of the time.” And especially among those for whom parenthood is wanted and eagerly anticipated.

I’m also curious to what degree all the members of a pregnant woman’s household experience heightened prolactin levels. For instance, could prolactin explain why my 21-month-old male toddler has been far less aggressive (decreased testosterone), much more affectionate, and suddenly interested in babies? He really has no intellectual comprehension of the fact that there is a baby in my belly or that a baby is coming to our family soon, so I know that can’t explain it. But over the last few weeks (particularly yesterday) I have watched him kissing, hugging, wrapping, and carrying baby dolls. It’s amazing to see how tenderly he will lay the baby down and cover it carefully with a blanket. All from a kid who is known to be “all boy” and has no trouble crashing toy cars or throwing all variety of objects across a room. Is he just re-enacting what he has seen his sisters do? Maybe. But all this research has got me wondering whether his behavior is motivated by shifting hormones as well. And it makes me wonder whether a hypothetical long-term house guest might feel especially invested in helping with childcare as a result of being in my proximity during pregnancy. It’s all fascinating to me, regardless.

Have you experienced a pronounced nesting urge? Have you noticed nesting behavior among those who live with you?