How Conception Works, and Where IVF Steps In

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7 minutes

We all know HOW pregnancies happen (*chuckle*), but most of us don’t know how PREGNANCIES happen.

Once you see the normal path from egg to pregnancy, it becomes a bit clearer where it can break down and how IVF helps when it steps in.

You’re born with every egg you’ll ever have

Egg cells are made before birth. When a female fetus is still in the womb, her ovaries build their entire lifetime supply of eggs. The peak comes around the middle of pregnancy with roughly 6 to 7 million eggs. She’ll never have that many again. The decline is immediate. By birth it’s down to 1 to 2 million, and by puberty about 300,000 to 500,000 remain. And across an entire reproductive life, only about 300 to 500 eggs are ever released. The rest are broken down and just reabsorbed.

This is why age matters so much in fertility. It’s not only that the number of eggs drop, quality drops too. The remaining eggs have been in a kind of suspended animation for decades, so older eggs are more likely to carry chromosomal errors. That decline speeds up after the mid-thirties.

Interesting shift in debate. The idea that females are born with all her eggs has been a cornerstone of biology for about 70 years. But in the last two decades, researchers found cells in the ovaries of mice and humans that appear, under certain lab conditions, capable of forming new immature eggs called oogonial stem cells. It’s still contested and unproven whether they can actually produce viable eggs in a living person. This doesn’t change current fertility medicine. Just fascinating new research.

Menstrual cycle

Every month, a woman’s body runs through a cycle to prepare for a possible pregnancy. This monthly cycle starts with your period. A couple weeks later, you ovulate: your body releases an egg, and the uterus has thickened its lining to prepare for a possible pregnancy. If no pregnancy happens, your period flushes away all those preparations.

The egg

We’re taught that during ovulation, your body picks and releases one egg a month. Yes and no.

At the start of each cycle, a hormone called FSH (follicle-stimulating hormone) recruits about a dozen follicles, the tiny fluid-filled sacs that each hold an immature egg. They all start growing, and then within about a week one follicle pulls ahead and becomes dominant. It keeps maturing while the rest stall out and are reabsorbed.

So each month, your body prepares a batch of eggs, one gets released, and the rest are let go. This is key to understanding IVF later.

Ovulation and the fertile window

When the dominant follicle is ready, a surge of another hormone (LH) triggers it to release its mature egg. That’s ovulation. The egg is swept into the fallopian tube, and waits there for sperm. It doesn’t wait long though. An egg is only viable for about 12 to 24 hours after it’s released. If it’s not fertilized in that window, it breaks down and the cycle moves on toward a period. But sperm are more patient and can survive for about 5 days. What actually makes conception likely is sperm being already there, waiting, when the egg arrives. So the “fertile window” is roughly 6 days. Timing around that window is the whole idea behind natural conception and simpler treatments like IUI.

Sperm meets egg: fertilization

After sex, sperm travel up through the cervix and uterus into the tubes within minutes. Of the millions that start, only a handful reach the egg, and only one gets in. Fertilization then happens in the fallopian tube, not the uterus. The sperm fuses with the egg, their genetic material combines, and the egg becomes a single new cell called a zygote. That cell already has a full set of chromosomes, half from each parent.

The journey down and implantation

The zygote doesn’t stay in the fallopian tubes. Over three to five days it travels down the tube toward the uterus, dividing as it goes: two cells, four, a solid ball, and finally a hollow ball of about 100 cells called a blastocyst.

Once in the uterus, it has to attach to the uterine lining in order to keep growing. That’s implantation. It’s roughly 6-10 days after fertilization. If it implants successfully, the cells that will become the placenta start producing hCG, the hormone a pregnancy test detects.

Many early embryos never implant at all. A fertilized egg failing to implant is common, and it usually passes as a normal period. Even in perfectly healthy couples, any single month is far from a guarantee. That’s part of why conception can take a while even when nothing is wrong.

Where it can stall

Now that you can see the whole chain, you can see every place a link can break. Infertility isn’t one problem. It’s any one of these steps not working:

  • No egg, or a lower-quality egg. Ovulation may not happen regularly, egg supply may be low, or egg quality may have declined with age.
  • Sperm problems. Too few sperm, or sperm that don’t move well enough to reach or penetrate the egg. This is a factor in roughly half of cases
  • Blocked or damaged fallopian tubes. If the tubes are scarred or blocked, egg and sperm can’t meet, or the embryo can’t reach the uterus.
  • Fertilization doesn’t happen. Even when egg and sperm meet, they don’t always successfully combine.
  • The embryo doesn’t implant. The embryo may not be viable, or the uterine lining may not be receptive.
  • Unexplained. Every test comes back normal and pregnancy still isn’t happening. It’s more common than people expect, and deeply frustrating.

Where IVF steps in

IVF (in vitro fertilization) is effective because it doesn’t fix the natural process, it bypasses the parts that tend to fail. Walk it back through the chain:

  • It rescues a whole month’s worth of eggs. Remember that every cycle your body recruits and grows a group of eggs. One pulls ahead and the rest get let go. IVF stimulation uses higher doses of FSH to mature several eggs from that same batch, instead of losing them. This also clears up a common fear: IVF doesn’t use up your future eggs faster. It collects eggs from the current month’s batch that were already going to be lost.
  • It collects eggs directly. In a retrieval, the mature eggs are gathered straight from the ovaries, skipping ovulation.
  • It handles fertilization in a lab. Egg and sperm are combined in a dish, bypassing the fallopian tubes completely. And for sperm issues, a single sperm can be injected right into an egg.
  • It grows and checks the embryos. Embryos develop in the lab for a few days, where they can be observed, and in some cases genetically tested, before transfer.
  • It places the embryo where it needs to be. In a transfer, the embryo is put directly into the uterus, skipping the journey down the tubes. Implantation still has to happen on its own, which is why IVF improves the odds at many steps but can’t guarantee the last one.

Seen this way, IVF is less a single procedure and more a set of workarounds, each aimed at a specific place the natural path tends to break. Understanding the natural version is what makes the rest of it make sense.

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The Frozen Egg is a simple guide to IVF treatment, written by a couple figuring it out themselves.