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Here is a link that might be useful: lamb's quarters

If you mean generally Chenopodiums (excluding Chenopodium ambrosioide) it is the leaves and seeds. C. Ambrosioide smells like varnish, so you probably won't eat it any way. BUT... long ago I had a friend call me and ask: How do you get the lambsquarter's seeds out of the black berry? Well, chinopodiums do not have berries and I knew immedately he was talking about the American Nighshade... he said they had been eating boiled leaves (of the nightshade) for a while and wondered why they were all getting headaches. The point is, make sure you have the right plant.

Have eaten my share of lambsquarter's, purslane and nettle, but when I say your post I had to reply! Berries? Not on the Chenopodium I know as edible... take it to the Cooperative Extension service ( a service of your land grant institution - there's one serving every county in every state) and have it positively IDed before you go munching. If it's nightshade -putting it on your cereal could be a truly BAD experience.

For some reason folks seem to get the common nightshade and the lambs quarters mistaken...whether the common nightshade is edible (and how and when) is a big debate. Cooked ripe berries might be edible. Leaves cooked in mor than one exchange of water may be edible. Though I am an expert in edible wild plants this is one area I have not personally explored. Maybe I should this year, to settle the issue. I have read a lot of repots that day the cooked parts are edible. Dr. Julia Morton, a south florida botanist wrote a book on edible plants in Florida. In reference to the solanum Americanum she wrote: Green fruits cntain solanine and are toxic; ripe fruits subacid, edible raw or cooked. ... Young leaves and stems cooked as greens. As I said I had a friend mistakenly eat the boild greens and it gave him and his family headaches.

I live in an area where lambsquarter grows like crazy. You must control it. When it seeds it will come up everyplace a seed falls. According to one University report, it will produce 100k seed and will come up for forty years.
I had lambsquarter come up throughout my garden this year.
I use the lambsquarter plant for a "green" drink. I cut a basket full, throw them into my Vita-Mixer with a half glass distiled water and "poof" I have me a green drink! I've also taken lambsquarter and steamed it. (Tastes like turnip greens).
I did see an article on Pigsweed & Lambsquarter and wrote the author, Sid Bosworth.

Here is what Sid said: "Yes, pigweed is classified as "poisonous" *but* it certainly isn't one I worry about too much. It is considered a potential nitrate accumulator.
If growing in a non-nitrogen rich soil, its not going to be an issue. Also, if mixed with other materials when eating, it would be diluted. I always warn livestock owners to watch if their animals, especially young animals, are eating weeds that are growing on manure piles. I find many of these to be pigweed or lambsquarter, both nitrate accumulators."
The article is located at:
http://www.uvm.edu/pss/vtcrops/articles/VTPoisonousPlants.pdf

I don't have any problem eating pigweed or lambsquarter, but if raised in an area that has manure, I wouldn't eat it.

I'm quite sure one variety of lambsquarters I have in my yard is Chenopodium album. (Click the link below for a really clear picture.)

About a week ago I collected some of the tops of these plants, including the leaves and the flower buds. I munched on a few -- very few -- of the buds. Probably a dumb thing to do -- in hindsight, anyway -- but they tasted kind of like cornsilk (which I like) and it seemed like a good idea at the time.

Anyway, I've been having some stomach pains off and on from about that day (or the next) on. Nothing that seems worthy of visiting the doctor yet, but enough to interfere with normal activity or at least be a distraction. No headaches or other pains, but some fatigue too.

Of course, it's hard to pin my symptoms on any one thing or event, since I don't have anything resembling a controlled experiment. It could have been something else I ate or even just some "bug" that's going around.

But I'm wondering if there's anything in the flowers or buds of lambsquarters (the variety shown at the link below) that would give you a tummy ache if you just ate about a teaspoon full of them.

Needless to say, I've suspended any further experimentation with the edibility of lamsquarters parts for the time being. From what I'm reading, it does sound like a good idea to blanch the leaves a couple times before eating them, not to consume mass quantities, etc. And, if you're going to eat the seeds, to let them ripen and dry, then thresh to separate them from the husks.

Here is a link that might be useful: Lambsquarters (chenopodium album) with flower buds