If baby is only breastfeeding, and not taking any formula or solid foods, it's very unlikely baby is constipated. It's quite common for breastfed babies past a month or two old to only poop once a week or even less. As long as the stool is not hard and dry when it does come, baby isn't constipated. Breastmilk is digested so well that often there's very little left over to be passed as stool. It's also quite normal for a baby to strain and turn red and pass a lot of gas and fuss, while trying to poop. That's because of immature muscle coordination, not constipation. It gets better with time, and baby will sometimes be helped if you carry baby face outward with baby's knees pulled up, or cycle baby's legs while baby is lying down.
If a baby on only breastmilk IS having hard, dry stools, inadequate hydration would be the first thing to look at. Baby may need to nurse more often. If baby exhibits blood in the stool or dark green or black stools, baby's intestine may be irritated, and you might consider an intolerance to casein, which means eliminating all dairy sources in your diet. But really, what you eat is unlikely to be a culprit, and adding something like prunes to your diet wouldn't help baby. What YOU eat isn't going to add any bulk to baby's stool or stimulate baby's intestine at all since it stays in your digestive system. All baby gets is the products of digestion that wind up in your blood stream, and thus in your milk, like simple sugars and amino acids.
If baby is on solids, you can try backing off on the solids, especially on stuff like bananas and cereal-based foods. You can also offer water, once baby is eating solid foods.
me , he , my three , , and -- and the one we lost