Pregnancy Week 3: Fertilisation, Cell Division and the Journey to Your Womb

Quick answer: In week 3 a sperm fertilises an egg in your fallopian tube, forming a zygote that divides as it travels towards your uterus, becoming a blastocyst of around 100 cells. Most women feel nothing at all. A test will almost certainly be negative now, as hCG only appears after implantation in week 4. Take 400mcg folic acid daily.

What parents need to know

Week three is a strange, quiet stage. If you're trying to conceive, you may be watching for any small sign that something has changed — and finding nothing at all. That's completely usual: almost everything happening now is invisible, and it's far too early for a test to tell you anything useful.

What's happening at week 3?

If conception has occurred, week 3 is where everything begins. A single sperm has penetrated an egg — usually in one of your fallopian tubes — and the two have merged to form a zygote. This tiny cell contains all the genetic information that will shape your baby: their eye colour, hair type, and even certain personality traits.

Fertilisation: the first 24 hours

Fertilisation typically happens within 12–24 hours of ovulation. The winning sperm fuses with the egg, triggering a series of rapid changes that prevent any other sperm from entering. The resulting zygote immediately begins dividing — first into 2 cells, then 4, then 8 — in a process called cleavage.

The journey down the fallopian tube

Over the next 3–4 days, the growing cluster of cells (now called a morula) travels slowly down the fallopian tube towards your uterus. Tiny hair-like structures called cilia help sweep it along. This journey is essential: the uterus is where your baby will be nourished and grow for the next nine months.

Becoming a blastocyst

By the end of week 3, the morula has developed into a blastocyst — a hollow ball of around 100 cells. The outer layer will eventually become the placenta, while the inner cell mass will become your baby. The blastocyst floats briefly in the uterine cavity before implantation begins.

What are you feeling?

Most women feel absolutely nothing different at this stage, and that's completely normal. The embryo is microscopic, and your hormone levels haven't yet risen enough to cause symptoms. Some women report very mild cramping or a faint twinge as the blastocyst begins to implant — but many notice nothing at all.

Is it too early to test?

Yes — at week 3, a pregnancy test will almost certainly show negative. hCG (the hormone detected by home tests) is only produced after implantation, which happens in week 4. Testing now is likely to give a false negative, so it's worth waiting a little longer.

What can you do right now?

Even though you may not know you're pregnant yet, there are things you can do to support early development: take a folic acid supplement (400mcg daily is recommended by the NHS), avoid alcohol, eat a balanced diet, and steer clear of smoking. These early weeks are crucial for neural tube development, even before most women know they're expecting.

What's next?

In week 4, the blastocyst will complete implantation into your uterine lining — and your body will begin producing hCG in earnest. That's when a pregnancy test will start to turn positive. Stay tuned for our Week 4 guide.