We all know that younger women, as a whole, have an easier time getting pregnant and giving birth to healthy babies. But understanding why that’s the case is more complicated.

Young Eggs vs. Young Uterus: What’s More Important for IVF Success?

We all know that younger women, as a whole, have an easier time getting pregnant and giving birth to healthy babies. But understanding why that’s the case is more complicated.

There are at least two different factors that make successful pregnancy trickier for older women:

  • Their eggs (that create the fetus) are older
  • Their entire body, including the uterus, is older

Let’s take a look at the impact of each on successful pregnancy. Then we’ll ask the question: which matters more – the uterus or the egg?

Why the Age of the Eggs Matters?

A baby girl is born with 1 to 2 million egg cells. That egg supply steadily decreases, giving the average adolescent 400,000 eggs and the average 37-year old 25,000 eggs.

Eggs don’t just decrease in quantity; they also decrease in quality, for several reasons. First, your body tends to pick the best quality eggs for use first – when you’re in your teens, 20s and early 30s. By the time you get to your later 30s and 40s, the remaining eggs are less likely to be as high quality.

Additionally, when the egg cell matures in the ovaries during the part of your cycle prior to ovulation, it needs to divide several times. Older egg cells are more likely to divide abnormally, leaving the cell with the wrong number of chromosomes. The result is an egg that will not fertilize, or – even if it does fertilize – will not develop correctly and will not lead to a successful pregnancy or a healthy birth.

Why the Age of the Uterus (and the Rest of the Body) Matter

Who hasn’t felt that her body at 40+ just isn’t the same as it was at 20+? (There’s a reason why women who don’t want to share their age often respond to “How old are you?” with the facetious answer, “21.”)

Women who get pregnant after 35 are more likely to experience pregnancy complications such as:
Gestational diabetes
High blood pressure
Complications during childbirth, including prolonged labour, need for a Caesarian section, or stillbirth

Any of those complications can decrease the chances of a healthy pregnancy and birth.

Which Matters More – the Uterus or the Egg?

Before we ask that question, maybe we should ask: Why does it matter which matters more? If you’re over 35, both your eggs and the rest of your body is over 35 – so who cares?

While you can’t change the age of your body (unless you’ve invented a time machine), when you’re using IVF you can potentially change the age of your eggs. This can be done either by using donor eggs from a younger woman, or by using your own eggs, if they were frozen when you were younger.

So, it makes sense to care about and to find out which matters more. Let’s take a look.

The UK’s HFEA (Human Fertilisation and Embryology Authority) presents some very telling numbers in their most recent report, using data from 2017.

IVF birth rates PET by age - 2017

The numbers tell about the live birth rate per embryo transferred (PET). For women using their own eggs, the birth rate steadily decreases, from 27-30% (depending on whether the eggs were fresh or frozen) for women under 35, to 2-4% for women over 44.

For women using donor eggs, however, the decrease is much less significant (where there even is a decrease). Birth rate per embryo transferred is 28-33% for women under 35 – not too different from the numbers for that age group who use their own eggs. But birth rate for women over 44 is 22-26%. What a significant difference from the 2-4% for women who use their own eggs!

Why is this the case?

Donor eggs are almost exclusively from women under the age of 35. When they use these younger eggs, even women in their mid-40s have a significant chance of having a successful pregnancy and birth. True, it’s still not as high as the success rate of those in their mid-30s, showing that the older body and uterus do have an impact, but it’s far higher than women who are using their own mid-40-year-old eggs, showing the much greater impact of the egg’s age.

Another support for the egg’s age having a greater influence than the body’s age is seen by difference in success rate between women using their own frozen vs. fresh eggs. For those under 35, the IVF success rate is comparable when using their own fresh eggs. All older age groups, however, have a higher success rate for frozen eggs. The eggs, frozen at a younger age, boost their chances of success.

The same boost is seen when performing IVF using embryos frozen at an earlier age.

The Fountain of Youth?

No, we don’t know of any miraculous source that will make you 21 again (if that’s even what you want). Your body is stuck at the age that it’s at. Period.

But the numbers given by the HFEA point the way to a potential fountain of fertility. Using younger eggs – whether your own frozen eggs or a donor’s eggs – can give you a higher IVF success rate. Additionally, the earlier you freeze your eggs (especially if you freeze them before 35), the greater your chances of a successful IVF pregnancy and birth.

We wish you much success in your fertility choices and in having the family you want, when you want.

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