I read an incredibly heartwarming story the other day that was first reported in the summer of 2017. A Nigerian couple were trying to conceive for 20 years with no success.
They would’ve been happy to have just a single precious child. They weren’t even considering how to conceive twins. Many in their position would’ve given up and simply accepted that it was never going to happen.
However, the couple still believed and were finally rewarded. Not with 1 child, not with twins but with six babies! I had to even check the name for six, sextuplets!
That got me to thinking, is there reason they had more than just one baby? Did their years of trying make twins more likely?
The Town of Twins
When I researched a bit more, I found that there are even towns that are famed for their high twin rate. Mohammadpur Umri, a small village in India, has a total of 33 pairs of twins, which doesn’t seem particularly high, until you learn that there are just 600 people living in this village!
Has anyone been able to discover the secrets to having twins? Why did the Nigerian couple have 6 children when they finally managed to conceive? Why is there such a high twin rate in some areas?
Based on our curiosity, we’ve found that there are range of reasons, some based on science, other not… We’re now going to detail some of things that actually influence how many babies you have and if there are things you can do to impact your chances.
However, to start with we are going to look at the actual causes for identical and non-identical twins.
What Causes Identical (Monozygotic) Twins?
This blog isn’t a science journal so I’m not going to go into the ins and outs of the theories surrounding identical twins. However, to add context to the rest of the article, I’m going to detail some of the most popular theories. You guys can choose your favourite and make your own decision.
Unbeknown to me, there are actually different types of identical twins. The zygote can split into two at any time during the first 10 days of the pregnancy. The timing of the split will determine the type of identical twins:
- 1 day – Dichorionic/Diamniotic – These twins have the time to develop their own amniotic sac and placenta.
- 4-5 days – Monochorionic-Diamniotic – These twins will have a separate amniotic sac but the late development will mean that have have to share a placenta.
- 10 days – Monochorionic-Monoamniotic (Mirror Twins) – This type of twins form too late to develop their own amniotic sac or placenta, so they share.
- 13-15 days – Conjoined twins – These are very rare, occurring once in every 200,000 births. Unfortunately, only 5 to 25 percent of conjoined twins survive birth.
In terms of what actually causes the zygote split, there is still some debate. A study in Lyon has suggested that it is the collapse of the embryo that causes the creation of twins.
During this collapse, the cells containing the genetic material are split in half, meaning that the genetic material is divided into small halves of the embryo, which eventually leads to the development of two fetuses.
However, from what I’ve read, while scientists have a good handle on the actual process that occurs, that are still pretty clueless when it comes to determining what actually triggers the process. Perhaps in the next couple of years, scientists will pinpoint the reason for this seemingly random event.
What Causes Non-Identical (Dizygotic) Twins?
Unlike monozygotic twins, fraternal twins are formed by the fertilisation of two different eggs by two different sperm. These zygotes will then share the womb during their gestation period. While these zygotes are sharing the same womb, they’re no more genetically similar than siblings born years apart.
The number of eggs fertilised and the number of babies created is dependent on the number of eggs that are released during ovulation. If 5 eggs are released and fertilised then 5 zygotes will be created.
Therefore, the underlying cause of twins and other multiple births is hyper-ovulation. Unlike the random split to create identical twins, there has been plenty of research and studies which have proven the causes of hyper ovulation:
Family twins: My mum had twins, does that mean I will?
There is really nothing that you can do about this one. According to the NHS, while identical twins don’t run in the family, non-identical twins certainly do.
Non-identical twins are a product of two separate eggs being fertilised. There is a specific genetic combination that makes it more likely for a woman to release two eggs during ovulation and that gene can be passed down.
That gene can also skip a generation if it is inherited by the man and then passed onto his daughters. If you have a fraternal twin yourself then you’re 2.5 times more likely to have a twin. This genetic propensity for twins is only important for the woman, the man’s family don’t impact on the chance of twins.
While there does appear to be a clear link between twins in the family and your chances of having them, scientists haven’t yet been able to pinpoint the specific gene that is responsible. It is likely that a combination of genetic and environmental factors lead to hyper-ovulation.
By contrast, for identical twins, the splitting of an embryo into two embryos is a random occurrence and hasn’t yet been proven to have any basis in your genetics. It does appear that this is pure chance.
It seems that there is a direct link between your BMI and your chance of conceiving twins. For women with a BMI of over 30, there is a significantly higher chance of conceiving twins.
According to the oft used scale, having a BMI of over 30 will mean that you’re obese with 25-29.9 being classified as overweight. However, this is often a crude scale that doesn’t take into account muscle mass and a range of other factors. Just because you have a BMI of 30 doesn’t mean you are obese!
It interesting to note that the rise in women’s average BMI in the past 20-30 years has led to an increase in the number of fraternal twins being born.
If you have a BMI of over 30 you’re not guaranteed twins but the evidence suggests you definitely have a higher chance.
The No. Children You Have
It seems that this one is more due to a combination of other factors rather than a direct cause. Although, if you want to see more opinions from mums then there is a great MumsNet thread on the subject. If you’ve already given birth to 4+ children then you’re likely to be significantly older, which means you are more likely to have twins.
If you have a large family then it is more likely that some of the children are twins, meaning you’re more likely. The average woman will have gained more weight from having such a large family, meaning she is more likely to have twins.
You can see that there are a combination of other factors that mean that having a large family correlates with a great likelihood of having twins, despite not actually causing it.
While this is a popular wives tale, there doesn’t seem to be a scientific reason for it.
After a height of 5 foot 4, the chance of conceiving twins grows exponentially with the increase in height. I used to think that this was a bit of a myth but there is conclusive evidence to support this theory. Women giving birth to twins are on average, an inch taller than those who aren’t.
It has been found that taller women have a higher amount of insulin-like growth factor. This insulin-like growth factor can increase a woman’s sensitivity to follicle stimulating hormones. This in turn will lead to the release of more than one egg during ovulation, increasing the chance of twins.
There is no doubt that the use of IVF increases the likelihood of having twins. By natural conception, the rate of twins is around 3 per 1,000. When using IVF, that rate climbs by some 7 times, up to 21 per 1,000. According to some figures, a whopping 40% of IVF pregnancies related in a multiple birth.
This is due to the number of embryos that are placed in during an IVF cycle. This is due to the poor quality of some embryos. This process means that there is significantly higher chance of a single embryo implanting.
However, it also means there is a greater chance of more than one implanting too.
With modern science, fertility doctors are now able to identify the embryos with the greatest chance of success. By choosing the high quality embryos, some are now opting for elective single egg transfer.
In this way, a single embryo is selected from your batch of embryos. Consequently, we may see the number of twins conceived via IVF drop in the coming years. The UK regulator has lowered the target for multiple births from 15% down to 10%, due to the potential complication.
According to the US Department of Health & Human Services, the twin birth rate has increased by over 75% since 1980. While there are a number of other factors, like the increased used of IVF, the average age of conception is a big part.
In the UK, the average age of first time mothers has climbed from 26.4 in 1974 to 30.3 in 2015 according to the office for National Statistics.
This trend is being mirrored in the United States:
There are a number of reasons for the increase in the average age of first-time mothers in the world’s most developed countries.
Women are choosing to have children later, more women have been able to build careers, there is less of a focus on having a large family and having children is more expensive than ever.
There is heated debate about the actual reasons but the evidence is undeniable. Women are choosing to have children later and later in their lives.
The impact of age on twin rate boils down to the number of eggs that are being released during each ovulation. As women get older, a higher level of follicle stimulating hormone (FSH) is required to trigger the release of the eggs. This increased level of FSH means that it is more likely that more than one egg is released.
In the US, an increased average age of first time mothers directly correlates with the increasing number of multiple births.
Since 1980, the rate of twin deliveries has climbed by over 52%, with over 100,000 twin births annually.
There have been a number of studies that have shown there are variations in twin rate among different races. In the US, while the overall twin rate between 1989-91 was 2.26%, it climbed to 2.69% for non-Hispanic blacks and fell to 1.67% in the non-Hispanic other group.
When looking at a global level, it is central Africa that has the highest twinning rate. Benin has a huge 27.9 twins per 1,000 births. In developing nations, the average twin rate is 13.6 per 1,000 births.
While it is clear from the research and the article so far, that there are dozens of factors at play, scientists haven’t yet pinpointed why there are such large variations in twin rates.
An Enzyme In The Sperm
While there hasn’t yet been proof of a gene that causes identical twins, there are some that have suggested that an enzyme in the sperm is responsible for the production of identical twins. Yes identical twins do seem to run in some families and some dads like to think they have super sperm but there isn’t yet the scientific proof for this theory.
Conceiving While Breastfeeding
There seems to be a real connection between the chance of conceiving twins and breastfeeding. One source suggested that the rate of twins rose from 1.1% to 12.4% when conceiving while breastfeeding. There is an interesting discussion on MadeforMums about the link between breastfeeding and twins.
This increased chance of conceiving twins is due to higher IGF (insulin-like growth factor) levels when breastfeeding. The higher IGF levels significantly increase the chance of more than one egg being released.
Again, this is another factor that I thought wasn’t true but it turns out there is a real link between dairy consumption and twinning rates. Those consuming dairy regularly are 5 times more likely to have twins. This is due to the link between dairy and insulin-like growth factor.
Consuming more milk leads to higher level of insulin-like growth factor, leading to increase sensitivity to follicle stimulating hormones. Increased sensitivity means more eggs are released during ovulation and there is a substantially greater chance of twins being conceived.
There are a couple of caveats to the main study that most of the articles link to. Firstly, those women with a lower dairy consumption could be deficient in other nutrients, particularly when following a vegan diet. Secondly, those women consuming more dairy, on average had a higher BMI. Finally, there has been contradictory research showing that the IGF levels in cows have virtually no impact on those in the milk drinkers.
Ultimately, it seems like there is some connection between the eating dairy and a higher twinning rate. Nonetheless, there are a number of mitigating factors.
Twins – Frequently Asked Questions
In researching this article, there were a few popular questions that kept cropping up and I though they should be included. I found them interesting, so hopefully you will too.
Just How Common Are Twins?
In 2015, of the 4 million live births in the US, just 24 involved 5 or more babies. To be honest, this is still a few more than I was expecting. While I think twins would actually be kind of ideal, there is no way I’m equipped to cope with 5 little ones running around. It would be an endless supply in crying, pooping and mess!
How To Tell If You’re Carrying Twins?
Twin pregnancy symptoms are not all that different to ordinary pregnancy symptoms but do often occur a lot earlier than would normally be expected. While there are a number of signs for a twin pregnancy, these read like a list of ordinary pregnancy signs. Plus, there is always the caveat that every woman is different and will have their own symptoms.
Weight gain, extreme tiredness, nausea and varicose veins are just some of the signs.
The only time you will truly know is when you are seeing two embryos on the ultrasound or hearing two separate heart beats.
Is Having Twins Dangerous?
It is true that there is an increased risk of complications with a multi birth pregnancy, when compared to the conventional single birth pregnancy. There are a number of issues that can be worsened by a multi birth. These include gestational diabetes, premature birth, low birth weight and preeclampsia.
The good news is that the health service are aware of the increased complications and so long as you’re regularly checked, a number of issues can be averted. There is no need to worry about having twins.