Breastfeeding vs. fertility

There is no clear cut answer to your dilemma. However, are some points that may help.

The biblical commandment of pru urevu is fulfilled by having one boy and one girl. There is a rabbinic commandment to continue to try to have more children – some sources indicate one additional boy and girl, others give no specific amount. Super large families are a modern phenomenon (the largest mentioned in the Torah from one woman is eight) – most likely due to a combination of factors including decreased infant mortality and decreased incidence of breastfeeding.

The Talmud permits continued breastfeeding for up to 5 years without worrying about the effect it may have on child spacing. This is despite the fact that the Talmud assumed that for at least two years after childbirth a woman was unlikely to get pregnant again. This would seem to indicate that the care of the existing child temporarily overrides a requirement to get pregnant again right away.

There is strong medical evidence that exclusive breastfeeding for 6 months and continued breastfeeding with complementary foods for at least one year and as long as desired by mother and baby is the healthiest way to raise a child.  Halacha would thus look upon this favorably. Unfortunately, there is sometimes a lag between medical knowledge and that which is known to rabbis. Then there is often a lag from knowledge on the part of rabbis to the ability to teach this to their communities in a way that will be broadly accepted. This has been the case, for example, with smoking prevention (although one can see the halachic progress). It is all the more true with breastfeeding, where there is a large marketing lobby for artificial infant feeds that has also watered down the medical message.