What determines the return of fertility while nursing?
If you read the older books about nursing and amenorrhea (such as Breastfeeding and Natural Child Spacing), the average time before a cycle returned was over 14 months.
This is assuming exclusive nursing on demand, no pacifiers, baby in bed, and a "nursing nap", which granted, not all moms do. But even among the moms who do all these things, the current average time before return of menses, based on the many women I've talked with, is about 9 months.
Some people do all of these and their cycle returns at 3 or 4 months postpartum, while others do only a few of these and get 18 months of amenorrhea. Some even have to wean entirely before getting pregnant. Mothers who had long amenorrhea while nursing have daughters who have much shorter spaces, so I wonder if overall caloric intake has risen, or if young moms now eat more protein than young moms 30 or 40 years ago when eggs had been maligned. What accounts for these wide differences? My first theory was that it depended on body fat. I noticed that the women whose fertility returned at 3 months postpartum despite following all the guidelines had generally been VERY thin and active as high schoolers. As adults they looked "normal", but this was 20-30 pounds over their high school weight. Knowing that fat is a hormone factory, I figured that if these women could run a cycle as a lean high schooler, there must be lots of hormonal energy to spare with the reduced activity and increased weight. The people who got the longest spans without a cycle were often people who had been sedentary and perhaps a bit overweight in high school. These people, like me, end up thinner as moms because they can't be sedentary. (Much as I'd like to read half the day.) Cycles tend to return when the baby is over a year. I figured this was because hormones didn't rise enough, what with being thinner and more active than during puberty, and having less body fat. It appears that no one really knows the answer to this question, and other theories involve prolactin sensitivity and overall caloric balance. I seem to get my cycle back when my baby/toddler is about 15-17 months and nurses less than 8 times in 24 hours, but I'm not sure if this relates more directly to prolactin levels or to that extra 1,000 calories a day or whatever it is that nursing requires. (500 always seemed a low estimate.) One interesting study from 2003 examined a group of well-nourished women whose lactational amenorrhea ranged from 6-14 months. All of the women were breastfeeding "intensively" (between 5 and 8 minutes per hour), and were part of the indigenous Toba in Argentina, who lead an agrarian lifestyle. These women were visited and observed for several hours a month, to calculate their overall duration of nursing and their overall caloric use based on their activity level. It was found that return of fertility tended to occur after 3 months of caloric surplus. Interestingly, these women became LESS active as their babies grew, and the researchers theorized that this was part of what created the caloric surplus, in addition to the introduction of other foods to the babies and the gradual reduction in nursing. Now, who among you is less active with a 9 month old than with a 3 month old? When my kids are 9-15 months, it takes just about all my waking moments to . ensure that they stay alive. They are very active and into everything, and I generally don't have other adults helping me keep them safe, because our communities are less tightly knit than those of the ladies in this study. I also don't like to eat when I'm tired, and I like to eat mostly real food, which may or may not be dense in calories. When nursing a newborn I eat huge amounts of butter, but fruits and vegetables and other basic foods don't add up the way you would think. So perhaps my caloric deficit lasts a long time. Also, while my kids enjoy sampling food as soon as they can grab it off the plate (around 4 months), they don't settle into real meals until much later. The researchers found that no way of slicing the nursing activity was predictive of how long amenorrhea would last. The mothers' age and number of children was not predictive. Introducing solids was mildly predictive. BMI, weight gain, and body measurements as a sign of extra calories was associated with the return of fertility. On average, women had 3 months of consistent "positive energy balance" before cycles returned. This is the "relative metabolic load hypothesis", and it makes more sense based on my experience and conversations with other moms. We have a well-nourished world where the limiting factors are just your time and interest in eating. My time and energy for eating, much as I enjoy it, tends to be limited until my babies are 15 months or so, and I'm on my feet a lot when they are 7 to 15 months. I've tended to be a little underweight when they are around a year old, probably a sign of "negative energy balance" or missing calories relative to my activity level. So perhaps that explains my long amenorrhea. If you've read this far, thanks! And what do you think? Does this mesh with your experience? Here's the study if you want to check it out: https://www.sas.upenn.edu/~valeggia/pdf%20papers/Valeggia%20&%20Ellison%202004%20Lactational%20Amenorrhea%20-%20J%20Biosoc%20Sci.pdf