My guest post on whether having lots of boys or lots of girls is genetic is up at What to Expect’s Word of Mom blog!

I am happy to share that I’ve written a guest post for the What to Expect Word of Mom blog. The post is about whether having lots of boys or lots of girls really does run in families – is it genetic?

Check it out here: 

I Tried a New Method to Find Out If I Was Having a Boy or Girl

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The genetic likelihood of having twins

Husband and I were talking one night recently about genetics and the likelihood of having a boy or a girl. As we were counting up the ratio of boys and girls on each side, I paused when counting my fathers siblings.

“He had two brothers and a sister,” I said. Then, “No, wait, 3 brothers and a sister.”

That was when I remembered – my father is a twin. It’s a fact I often forget because unfortunately, his brother died as a baby.

That’s when the question dawned on us, are we more likely to have twins? It’s something that I’ve been told is possible since I was a child, but which I forget, often. Again, because of the whole never having met or heard stories about my father’s twin. Because my dad doesn’t remember much more than what his picture looked like. I think I had only seen the picture once, but luckily just the other day my father sent us many pictures he had scanned in, including the one of him and his twin.

So we began investigating the question – are we more likely to have twins because my father was one?

Do twins run in families?

First, it depends on if you are talking about identical or fraternal. Identical twins are the result of the zygote dividing in two. Scientists don’t know much about what causes this, but as the rate, about 1 in 285 births, has remained constant over time and around the world, it seems to be a random occurrence. It does not occur because of genes.

But what about fraternal twins?

Fraternal twins occur when more than one egg is released at ovulation and fertilized. When this happens it is called hyperovulation. Hyperovulation can be genetic, though it can also occur in women with no family history of twins, particularly if the woman is older or on fertility medication.

Since ovulation patterns are determined by the woman’s biology, fraternal twins must run in the mother’s family for there to be a genetically increased likelihood of conceiving twins.

If fraternal twins run in the father’s family, then any daughters he has have an increased likelihood of having twins themselves. And that leads to the sense that twins skip generations.

So if my father was a fraternal twin, then he was no more likely to have twins than anyone else, but I am more likely.

I’m not sure though, if his twin was identical or fraternal. He may not even know for sure, as they obviously wouldn’t have done genetic testing in the 1930’s, so back then fraternal twins that happened to be very similar could be mistaken for identical twins.

Therefore even with the aid of the picture, we may never be certain if he was a fraternal or identical twin. There seems to be a very strong family resemblance among my dad and his siblings. My father’s two older brothers looked like twins as children, but they were not.

My father and his twin

My father and his twin

There's a strong family resemblance among my father's older brothers

There’s a strong family resemblance among my father’s older brothers

My husband is convinced there are differences in the two faces in the picture of my dad and his twin brother – mainly around the width between the eyes and a little bit in the mouth. But even if that’s true, I recently read that identical twins can have differences in facial features and other aspects, because of environmental factors affecting gene expression that impact growth even in the womb.

So, as for me, the jury is out. I may have an increased chance of conceiving twins, but we won’t know for sure unless it happens some day.