A few thoughts... Your premise, that HG societies would want to delay pregnancy or reduce the possibility of a pregnancy, is faulty. Maternal mortality was a significant issue, infant and childhood mortanlity were huge issues. Aside from family, there was no one to take care of a person in old age save their children. Additionally, more bodies to do work meant more resource accrual.

Chances are that, at least in an evolutionary environment, pregnancy was typically not something that one wanted to avoid. Pretty much ever. Lactational amenorrhea does the trick for the short term - in a modern environment, it's considered to be effective birth control for six mos as long as baby takes no supplements, as long as all suckling needs are met at the breast (no fingers or pacifiers), baby nurses at least every 4 hours (? I think that's the number) at night and on cue throughout the rest of the day.

For a mom dealing with scarce food resources herself (think EEA) and/or low-body fat stores, chances are she'll get more pregnancy prevention for a longer period of time.

My fertility returned when my older child was 18 months and when my younger child was 14 months. The younger one nursed a lot less.

Dear god! Lemon juice? Lysol?! What the...?

Please look into Fertility Awareness Method (Toni Weschler), I'm sure even uneducated HG know enough about their bodies so they can prevent children if they wanted to. Basically, check cervical fluid and cervix position and opening, to know if she's fertile. Also basal temperature, but I'm sure they didn't have thermometers back then. More than half of her menstrual cycle, she is infertile; no chance of getting pregnant. It's really not that difficult to avoid sex for the 5-10 (*) fertile days of the cycle, though it is more likely H-G families would want children, so... :-)

Though to be honest... those 5-10 (*) fertile days practically make you really horny, I bet HG women just assaulted their good looking men (oops, sorry, am I drooling?)

(*) depends on each particular cycle

I know that the ancient greeks used to submit the child(newborn) before the father who would then give his approval or disapproval in keeping it. If the latter...it was 'exposed' on a dunghill on the outskirts of town much as in modern day ghettos. Population control in the old days was coldly efficient.

From Dr. Sears:

Breastfeeding can suppress ovulation and thus serve as a form of birth control. When you breastfeed, the same hormones that make milk, called prolactin, also suppress the release of hormones that cause eggs to mature and become fertile and the lining of the womb to nourish fertile eggs. This is why breastfeeding mothers usually notice a delay in the return of their menstrual periods. This delay, called lactational amenorrhea, or LAM, may be nature's way of telling you that one baby is all your body can handle right now, and it's too soon for a sibling. As a natural method of birth control, LAM's effectiveness rate can be as high as 98 percent (similar to artificial methods), but only if you observe the following practices:

Breastfeed frequently. In order for the level of milk-making hormones to stay high enough to suppress fertility hormones, frequent, unrestricted breastfeeding is necessary. This usually means breastfeeding at least every two to three hours during the day, or as often as baby needs.

Night nurse. Since fertility hormones tend to be highest during the sleeping hours, it's also important to keep the milk-making hormones that suppress ovulation high at night. Since co-sleeping babies usually breastfeed more frequently during the night, having your nursing baby nestle right next to you during the night is one of the most effective ways to delay fertility. Once your baby starts sleeping through the night, the fertility hormones take over and breastfeeding is no longer likely to lead to natural child spacing. If you plan to train your baby to sleep through the night, your fertility is likely to return soon.

Avoid supplemental bottles and pacifiers. To keep the milk-making hormones high enough to suppress ovulation, it's important that all of baby's sucking be at the breast.

Delay the introduction of solid foods. Wait until baby is at least 6 months to introduce solid foods, and then make them an addition to, not a substitute for, breastfeeding. If you notice your baby is decreasing the number of nursings once you start introducing solid foods, back off a bit on the solids.

I don't know what hunter gatherers do/did, but I once did research to find out what courtiers and other ladies-of-ill-repute used to do. One of the most common forms of birth control besides make shift female condoms and diaphragms was lemon juice. At a 30% concentration mixed in water, it kills 100% of all sperm it comes in contact with. (Don't do more than 30%, as that becomes too acidic and can cause unpleasant damage.) So, ladies used to douche with lemon juice water before sex and immediately after. Also, they made their own spermicidal sponges using diaphragm-shaped sponges soaked in lemon juice water, being sure to insert them just before intercourse. Or they sometimes used half a lemon as a cervical cap, creating both a barrier AND spermicide from the lemon. (Although this seems like it would be VERY acidic since the lemon juice isn't diluted. Maybe it didn't hurt b/c the juice stayed mostly in the fruit?)