My son was sleeping for about 8 hours a night at 5 months and was using a pacifier, which I think contributed to the early return of my period. (breastfeedtampa.com)
Fertility / return of periods aside it is perfectly possible to continue night time feeding an older baby or toddler whilst pregnant or with a newborn. (sarahockwell-smith.com)
If you continue to do so for an extended amount of time, you will likely experience weight changes, the return of your period, and maybe even another pregnancy, all of which can wreak havoc on your hormones. (romper.com)