.... each woman is different. Each successive pregnancy is different.
there is no predicting it or consequent pregnancy timing.... nor the consequent return of a period after having a baby and/or while nursing or not. Also, you do not know how breastfeeding will go... or how baby or your milk will adjust to it.
I nursed my 1st child. I got my period even while I was still breastfeeding. I got pregnant while still breastfeeding, which ended as a miscarriage.
Then by the time my daughter self-weaned from breastfeeding... and after the miscarriage... and per the Doctor's recommendation as to post-conceiving after a miscarriage/DNC... I had gotten pregnant with my now son... about maybe 7 months after that.
It all really depends. There is no formula to it.
MANY women get pregnant while breastfeeding. But, keep in mind that if pregnant while breastfeeding... it changes the taste & constitution of your breastmilk... and some babies will then "reject" nursing. OR... your breasts become SO tender/sore from being pregnant... that you may not want to breastfeed anymore. Or, you will get much morning sickness and discomfort while pregnant.. and then breastfeeding becomes unbearable.
There are many scenarios... which are not predictable.
With 2 babies close together in age... you will not really know if you will want to do that... AFTER you have your 1st baby... because sometimes a Mom changes her mind, because after having a 1st baby, sometimes the fatigue is too much and lack of sleep... or you may want to enjoy and spend time with your 1st baby... more... until they are older. Or some Moms just want to have all their kids close in age... and be at the same stages with diapers, wakings, age stages etc. Which sounds like what you are maybe thinking. But... having kids/babies all soo very close in age... is not a piece of cake. And it does not guarantee, that the siblings will be closer or get along, better. It is all about personality dynamics... not numeric age differences.
Once you have baby... and with breastfeeding, it is strongly recommended to STILL take your pre-natal vitamins.... and for conceiving as well. So, your intake of vitamins and healthy eating, is still at the forefront. And a priority.
Just make REAL sure... that you want to get pregnant so soon after having your first baby.... being "under 6 months old" as you said. MANY Ob/GYN's recommend waiting.... a woman's body, post-birth, takes about 1 year to return to normal and "recover" from a pregnancy and birth.
Sure, many women get pregnant soon after having a baby.... and they are fine. So you have to evaluate that, and at your check-ups post-birth, with your Doctor.
Many aspects to this. Think it out.
No pregnancy or birth, is as we theoretically think it will turn out... but plan ahead... and once your 1st baby arrives... then it will give you a clearer window, into what you will decide.
All the best,
Susan