I'm a bit confused...

I’m guessing it’s becaus he is such a weak omega

Eh i though all omega easily get pregnant if sex during heat period?

Think cause he's a male omega. Usually omegas are females, but male omegas are more uncommon.

He is different , for example he knew that he was omega when he was already adult unlike most omega.

There are 2 versions of omegaverse lore. One is that when in heat the omega can get pregnant easily. The other is like this manga, where omega fertility isn't great.

Thank you guys ^^

I thought it was because he is a "recessive omega"? I think this is the first omegaverse comic in which there's been a clear distinction between dominant and recessive omegas and alphas.

Nope. There aren't only two. Every omegaverse story is actually unique upon itself. So there's like hundreds of versions of omegaverse out there. The only rules they all have in common, and I guess for them to be considered omegaverse, are that males can get pregnant, and the existence of the A/B/O categories.

everyone else here that replied, the answer is "because he is a ressesive omega". It is explained in the first few chapters. Male omegas & female both have higher fertility rate than beta, especially when in heat. He is a "weak" omega, just like the other alphas in the story, unlike MC, are "weak" alphas . Meaning the sperm count & pheromones [or in case of omega healthy receptive ovum & fertile uterus to plant embryo in] is better with stronger , non-ressesive genders. Hope this is clear and resolved now?

The alph used condom, so the omega here can’t be pregnant

To be fair, not even all omegaverses have betas! :P
That reminds me, has this story brought up the whole “neck biting/partner” thing yet? That’s one of my least favorite tropes.

Ok if it's anything like regular genetics an omega can only be a recessive. If they were a dominant you'd never have a dominant alpha born. The only way you'd get a dominant omega is if 2 omegas had a child.

It could be a separate gene kind of thing. For example, maybe a dominant alpha and dominant omega produce dominant alpha/omega offspring. And maybe even with one recessive parent, offspring can be either dominant or recessive.

I’ve tried to think up solutions to this for a while... (I’m a bio student)
Perhaps ‘alpha’ and ‘omega’ are two separate genes: so the alpha gene and the omega gene. AAOO would be beta, AaOO would be omega (dominant), AAOo would be alpha (dominant), aaOO would be omega (dominant) AAoo would be alpha (dominant). AaOo would be beta, Aaoo would be alpha (recessive), aaOo would be omega (recessive), and aaoo would be beta.
So if you got an omega gene from one parent (O) and not one from the other (o) and not an alpha gene from either, you would be a recessive omega?
So two heterozygous betas (AaOo) would have a 4/16 chance (simplified to 1/4) of a recessive alpha or omega child.
I dunno. That’s the way that makes sense in some weird way, but I gotta do more work with it.

This is yaoi; I don't think logic and science can be applied to this story. I think the terms "recessive" and "dominant" are used in this context in a non-scientific manner, as in being a "recessive" omega signifies that one is "a late bloomer" with very low chances of getting pregnant (fertility issues); correspondingly, I think the author uses the word "dominant" to signify that the seme has no issues whatsoever in terms of impregnating an omega (dem reproductive powers, aarrghh). This is the first omegaverse manga where I've ever seen these biological terms being (mis)used - it's obvious that the author has no idea about the meaning of these terms when used in a scientific context. Yaoi is ... as we're all well-aware ... not very .. uhmm, scientific. (￣∇￣") Getting caught up in the irrational logic and science of the author's made-up universe won't lead anywhere, I'm afraid. (▰˘◡˘▰)

I realised that logic and no place here when I posted it. If logic ever appears in the Omegaverse I will need a lie down from shock. But an enjoyable read and I can't get enough of the Omegaverse.

Lol I think you've done plenty already. But I was thinking simply of Mendelian genetics and with no Beta to confuse the issue. So a Dominant/Homozygous Alpha would be AA and a recessive /Heterozygous Alpha would be Aa. And the same with the Omegas with a dominant/ Homozygous being an OO and a Recessive/Heterozygous being an Oo. But fertility questions then crop up again. It's a long Easter weekend where I have time to think about this. Btw I breed dogs, so it's why I have that interest.