There is a study in sexing eggs by shape.

[NatJ]

I don't know, since the whole egg is so important not just the yolk it seems reasonable that it would be involved

The egg is important for the chick to grow.

But the sex is determined by the genes of the chick, which start as just one fertile cell on the surface of the yolk. By the time the egg is laid, the "chick" is a bit more than a single cell, but is still so small it should have no effect on the shape of the eggshell.

There are several ways to know that sex in chickens is determined by genes and not by egg shape or incubation temperature or anything of the sort. One way to tell: sexlinks work. Certain crosses of male x female will give chicks that have males of one color and females of another color. For example, a Rhode Island Red rooster and a Barred Rock hen will produce daughters that are black, and sons that are black with white barring. The barring goes from mother to son, never from mother to daughter. This reliably does work, which is only possible if the sex is determined by genes, not by any other factor.

 

[No Coop No Problem]

But the sex is determined by the genes of the chick, which start as just one fertile cell on the surface of the yolk. By the time the egg is laid, the "chick" is a bit more than a single cell, but is still so small it should have no effect on the shape of the eggshell... This reliably does work, which is only possible if the sex is determined by genes, not by any other factor.

I figured the shape of the egg was downstream of genetics. In other words, the mother hen creates the cell that is either male or female, then the egg is shaped accordingly for some presently unknown biological purpose

 

[janell12]

I figured the shape of the egg was downstream of genetics. In other words, the mother hen creates the cell that is either male or female, then the egg is shaped accordingly for some presently unknown biological purpose

Agreed. I have a hen who lays an almost round egg, always. And one who lays a skinny pointy egg, always. This would mean they’ll lay a female or male, always. This summer I'll be using those roundish eggs for all my broody hens just to experiment!

Another point from the study: it said they sexed the chicks at 1 week old depending on body size, comb color and feathering. I wonder how accurate this really was for the breeds of chickens they hatched. I’d like to see the same study use vent sexing since that’s industry standard for hatcheries. Or hatch sex linked eggs so they know what sex the day old chicks are.

 

[arazla]

I read the same research and although it was a small sample, I decided to give it a try. I got 5 HENS and 1 roo. But I didn't let my broodies hatch many, so I may have just gotten lucky. The problem is as some have mentioned, that each hens tends to lay a certain shape. I tried to study their shapes and choose the roundest ones from each girl, but got confused after a while, or some got soiled and had to be replaced etc. The roo hatched from a fairly round egg, but I lost track so had no way of knowing now which hen it was from and whether it was a more oblong or pointy one for that particular hen. Nevertheless, since I got so lucky, I'll keep using this method.

 

[McChooky]

The study results can be read at the link below. However, just a warning... It gets very technical and may make your eyes roll to the back of your head.

In short, the results say, "Yes. You can determine chicken sexes by shape" with some 80ish percent accuracy.

Imagine the thousands of culled male chick's that could gave been saved from their terrible end if this method was used more and used correctly.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC9832119/#:~:text=In general, eggs with a,index will be female chicks.

I've read some wild birds can tell what sex the eggs will produce beforehand.If they need more males they'll hatch those but I don't recall where I read it