A scientific study determines how likely you are to conceive all the children you want – with or without IVF – depending on when you start trying.

Not everyone wants children. But those who are certain that they do usually have an idea of how many they want, too. And typically, that’s more than one: The average size of a completed family in England and Wales is 1.89 children per woman.

Generally, we know the status quo when it comes to our ability to conceive: women’s fertility gradually reduces in their thirties. “Women’s fertility starts to decline in their mid-thirties, the lowest fertility rate being in women in the over 35 age group,” explains Dr Shabana Bora, Consultant Gynaecologist and Fertility Specialist at HCA UK’s Lister Fertility Clinic. “Even in fit and healthy individuals the total number and quality of the eggs decline more rapidly in these women. The rate of decline in fertility rates is gradual and the speed at which it occurs varies, primarily due of a combination of genetic and lifestyle factors,” the doctor adds.

Based on the above, we’re all pretty clear on when it’s advisable to start trying for a baby in order to establish a first pregnancy. One thing that’s perhaps less publicly understood, however, is how this age might change if you intend to conceive more than one child. But it turns out that a scientific study dating back to 2015 addresses this head on, and its conclusions make for some interesting reading if you’d ideally like to have two or more children in future.

The research, which was conducted by Erasmus MC researchers and published in the Human Reproduction journal, created what’s effectively a mathematical model that can be used to help would-be parents work out when they should ditch the contraception. Using a computer simulation model of fertility, along with data from IVF success rates, the experts simulated a cohort of 10,000 couples in order to assess the chances of realising a one-, two- or three-child family at different female ages, with or without the aid of IVF.

So let’s get to the most important bit: What age should you start trying for a baby if you hope to conceive three children without IVF? Don’t be alarmed at the stats – they’re percentage based on likelihood, and should be treated as a guideline rather than guaranteed fact. Every body is different, after all.

According to the study’s conclusions, if you want a 90% chance of conceiving three children without needing to use the aid of IVF, the data suggests you should start trying for a baby at the age of 23. Yup, we know that sounds young – especially considering the average age of first-time mothers in the UK is currently 28.9 years old – but hold on. Don’t start panicking that you’ve already left it too late. For a 75% chance of conceiving three children without IVF, the data suggests starting aged 31, and for a 50% chance of having three babies without any fertility treatment, you’d need to start trying at 35.

With the assistance of IVF, those ages get pushed back. For a 90% chance of conceiving three children via IVF, the research advises starting at 28. For a 75% chance, it would be 33, and for a 50% chance, it would be 36.

But what if you only want two children? Unsurprisingly, according to the study, that buys you more time. For a 90% chance of conceiving two children without fertility treatment, the research suggests starting at 27. For a 75% chance, you could wait until 34, and for a 50% chance, you could start trying at 38.

Take a look at this table to see the study’s conclusions in full:

But as previously mentioned, this shouldn’t be taken as gospel because everyone’s body works in different ways. Some will find conception very straight forward, while others might struggle. Generally, though, “it’s estimated that 90% of couples will conceive within 12 months of actively trying to conceive,” says Dr Shabana. “For women under the age of 35, if they have not achieved a pregnancy in the first 12 months, they are advised to seek advice and start investigations into reasons as to why they haven’t been successful (both male and female partners). For women over 35, however, we would encourage them not to wait 12 months and get the ball rolling after six to avoid wasting precious time,” she explains.

Ultimately, we never really know how our fertility is until we start trying. But the most important thing is to make sure you start trying because you feel ready, and it’s what you and your partner want. While studies like these are useful for gaining a realistic perspective, don’t let it rush you if you’re not quite ready to jump into parenthood yet.

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