Cate Shanahan of Deep Nutrition has been thinking about this for sometime now. http://drcate.com/

I was still nursing my almost 3-year-old at the time I conceived our second child (no birth control was used in the interim, so I assume this was the spacing my body found acceptable). I was a little worried about the partition of nutrients thing, but most of what I researched seemed to indicate that the fetus would take priority, and that it was perfectly safe to keep nursing unless the stimulation was causing contractions. For some reason the nursing seemed to lessen the morning sickness a bit too. I did notice that several years into the breast feeding relationship that the nutritional toll seemed to have decreased significantly, and I had adapted to to nursing 6-8 times per day on less than 2000 calories with no problem with supply, and even putting on weight. I think our bodies might be clever enough to make these things work, and be pretty darn protective of viable embryos.

I don't have any personal tandem nursing experience because my older child self-weaned about 3 months into the pregnancy and the second one is still cooking. I have a number of friends who nursed through their second pregnancies, and continued with tandem nursing of 3 and 4-year-olds along with the newborn without any obvious problems other than the exhaustion of extra feedings, and a bit of jealously from the older child.

Ashley~ FWIW, you don't sound out of the norm at all for breastfeeding, compared to the moms I work with who regularly do baby-led weaning & tandem nursing (and you sound completely normal, compared internationally.) Katherine Dettwyler is considered the expert here: http://en.wikipedia.org/wiki/Kathy_Dettwyler

I don't have any references, but the 3-4 year birth interval in H/G cultures is also what I've read. I believe that abstinence from intercourse (though not sex) contributes to this interval, along with lactation amenhorrea in the first year.

I suspect that the interval could safely be shorter, from a nutritional standpoint, if mom's gut health is optimal, she started out her 1st pregnancy well-nourished & she has access to nutrient-dense foods year round.

From a child development standpoint (and to reduce mom's stress level!), I get that 3-4 year spacing may be optimal.

I fed all 5 of ours including twins for 1 - 2 yearts but all gaveup feeding . I never gave up on them. I would have been happy to do it longer. Our birth intervals were 2 year 2 months 1 year 8 months

The twins were after a 10 year gap

Having the 3 children under 4 was quite hard work but now they are a lovely unit with lots in common. There are lots of social reasons to have chidlren about 2 years apart and it can make child care easier and it can also means if you both work full time as we did things are more efficiently done.

I also read that a natural spacing in some older societies was about 4 years but my fertility always returned after about 4 - 6 mnoths even though ours all woke every night for 3 or 4 years.

I would have liked to have tried tandem nursing and am an LLL member etc but none carried on beyond 18 months to 2 years old breastfeeding. If you can feed twins as I did exclusively on breastmilk ie make enough for two you can do the smae for a baby except I think the composition of the milk changes in pregnancy - some toddlers don't like the taste and also I suspect what you produce for a new born is different from what is fed to a toddler from the breast.Certainly worth trying if you want to. I am probably glad I did not as it would have been an extra demand really.

I became pregnant with my second daughter when my first was 9 months. I continued to nurse throughout my pregnancy and tandem nursed for a year. My youngest weaned when she was 18months. I know it's just one woman's experience, but my first daughter was 7lbs, my second was almost 10lbs. She didn't seem to be missing out on anything in the womb due to my continued breastfeeding. I didn't experience any contractions during my second pregnancy due to breastfeeding. My doctor wasn't worried about it, although everyone else I know seemed to be concerned. Both my girls were fine.

An interesting thing about tandem nursing...it really helped when my youngest was born. She was having some tummy issues from what I think was too much foremilk. She also struggled because my letdown was too strong. Both of these issues were fixed my having my oldest breastfeed first for about 5 minutes before my younger daughter did. My letdown wasn't as strong after she nursed and my youngest got the hindmilk that helped her tummy.

Have you asked the La Leche League? Do you have access to a trained midwife?

Just a data point for you, my kids are almost exactly two years apart, and were breast fed for 18 and 20 months. Like you said, outside the norm, but this worked great for both kids and mom, and at ages 7 and 9 my kids are super healthy, and i attribute some of this to the extended breast feeding.

What stopped my wife from breast feeding the first kid when she was pregnant with the second was some spotting and hormonal imbalances. The first kid was 15 months old when she got pregnant. She kept breast feeding, but when she was about 3 months into the pregnancy, she got some spotting which her midwife suggested might be due to the breastfeeding, and we weaned the first kid, and sure enough the spotting stopped and everything proceeded normally.

I am not aware of any studies sugggesting ideal intervals or anything like that, but it seems like common sense that trying to breast feed one kid while pregnant with another is tricky. I am sure it is doable in a pinch and might be common in primitive cultures, but if you have the choice you would probably not do it.

The most important feature for breastfeeding to be contraceptive is breastfeeding at night. The recommended interval between children is a MINIMUM of 3 years, ideal is 3-6 years.

Another good book to read is Healing Our Children by Ramiel Nagel.

When you become pregnant the breast milk changes taste to encourage the previous child to wean.