Dr. Aruna Ashok MBBS, MS OG, DNB OG
- Clinical Director
Aruna Ashok | 24 June 2025
If you're trying to conceive, you may have come across a lot of talk about egg size—especially if you're working with a fertility doctor or tracking ovulation. But how big does an egg need to be for pregnancy actually to happen? What is the minimum egg size to get pregnant?
Let's explore this topic in simple, clear terms so you understand what's going on inside your body and what it means for your fertility.
Before we get into sizes and numbers, let's understand what we're talking about.
Inside your ovaries are tiny fluid-filled sacs called follicles. These follicles each carry one immature egg. During each menstrual cycle, usually, one follicle is selected to grow and mature.
As it grows, the follicle fills with fluid and nourishes the egg. When the follicle is fully mature, it releases the egg in a process called ovulation.
So when doctors talk about "egg size," what they're measuring is the size of the follicle, not the egg itself. However, since follicle size closely reflects the maturity of the egg inside, it's a key factor in determining fertility.
The egg inside the follicle must be mature for it to be fertilized by sperm. A mature egg has the correct number of chromosomes and is capable of joining with sperm to create an embryo.
If the follicle is too small, the egg inside is likely immature—and immature eggs usually can't be fertilized or won't result in a healthy pregnancy.
So, knowing the minimum egg size to get pregnant is important, especially for women tracking ovulation or undergoing treatments like IUI (intrauterine insemination) or IVF (in vitro fertilization).
The short answer:
The minimum follicle size for a mature egg that can result in pregnancy is about 18 mm to 20 mm in diameter.
Here's a breakdown to make it more straightforward:
| Follicle Size (mm) | What It Means |
|---|---|
| Under 14 mm | Too small – egg likely immature |
| 14–17 mm | Growing – may mature in the next few days |
| 18–20 mm | Ideal size for ovulation and conception |
| 21–24 mm | Still okay – egg is mature |
| Over 24 mm | Possibly overripe or may not release properly |
Doctors use a transvaginal ultrasound to track follicle growth. This scan shows the size and number of follicles in your ovaries.
If you're undergoing fertility treatment, these scans are usually done:
Once a follicle reaches about 18 mm or more, your doctor may recommend:
Anu, a 31-year-old woman from Coimbatore, had been trying to conceive for over a year. Her doctor started her on ovulation induction medication and monitored her cycle.
On Day 10, her scan showed a 14 mm follicle. By Day 13, it had grown to 18.6 mm. Her doctor gave her a trigger shot, and she and her husband were advised to try for a baby over the next 2–3 days.
Two weeks later, Anu took a pregnancy test—and it was positive!
Her story illustrates why tracking the minimum egg size required for conception can be particularly helpful when trying to get pregnant.
If the follicle is too small (less than 14 mm), the egg inside likely hasn't matured yet. Ovulating at this stage might release an immature egg, which:
In most cases, the body naturally waits until the follicle is big enough before releasing it. However, in some women, early ovulation or hormonal imbalance may lead to smaller follicles releasing immature eggs.
It's rare but not impossible.
There are occasional cases where women conceive even when the follicle is slightly under 18 mm, especially if:
That said, the chances of pregnancy go up significantly when the follicle is 18 mm or larger, as that's when the egg is most likely mature.
There are a few ways to support healthy follicle growth and egg maturation:
Always consult your doctor before starting any supplements.
Yes—very much!
In IVF, doctors aim to retrieve mature eggs directly from the follicles. The minimum egg size to get pregnant through IVF is still about 18–20 mm.
In fact, during IVF:
Sometimes, even if 10 eggs are retrieved, only 5–6 may be mature and usable.
Follicles that grow too large—say, over 24 mm—might contain overripe eggs, which:
That's why timing is so important. Doctors try to retrieve or trigger ovulation when the egg is mature—but not overripe.
1: Can I ovulate with a 16 mm follicle?
Yes, but it may be too early. The egg might still be immature. If you ovulate naturally at 16 mm, the chance of pregnancy is lower.
2: How many mm should a follicle grow each day?
On average, a healthy follicle grows about 1 to 2 mm per day during the growth phase.
3: How many mature follicles are needed for pregnancy?
Just one healthy follicle and egg are enough for natural conception. For IVF, doctors usually aim for 8–15 mature follicles to increase success chances.
4: Is egg size the only thing that matters?
No. Egg quality, chromosomal health, sperm quality, and uterine environment all play a role in getting pregnant.
Understanding the minimum egg size to get pregnant can be a game-changer when you're trying to conceive. Whether you're tracking ovulation naturally or going through fertility treatment, knowing that a follicle should ideally be at least 18 mm before ovulation gives you clarity and confidence.
But remember, egg size is only one part of the puzzle. With the proper support—nutrients, timing, and care—your body can do amazing things.
