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

6 min read

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 usual: almost everything happening now is invisible, and it's far too early for a test to tell you anything useful.

Fertilisation timing	Within 12–24 hours of ovulation
Journey to uterus	3–4 days down the fallopian tube
Size by week's end	Blastocyst of around 100 cells
Pregnancy test	Almost certainly negative this week
Folic acid	400mcg daily, recommended by the NHS

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.

Close-up of a parent's hands gently holding the feet of a newborn baby, capturing a tender moment.

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. 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 nothing different at this stage, and that's 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.

When to get help

Some symptoms in pregnancy can be a sign of something serious and need checking quickly. This is who to contact, and how soon.

Call 999 if:

you're bleeding from your vagina and the bleeding is heavy (soaking a period pad soon after putting it on), or you also have severe pain in your tummy, pain in your shoulder, or feel sick, faint, dizzy or lose consciousness
you have a sharp, sudden and intense pain in your tummy together with feeling very dizzy or fainting, feeling sick or looking very pale, as this could mean an ectopic pregnancy has ruptured (you can also go straight to A&E)
you have severe difficulty breathing (you're gasping, choking or not able to get words out) or your chest feels tight or heavy
you have pain and swelling in one leg and you also feel short of breath or have chest pain. A blood clot in the leg can travel to your lungs, which can be life-threatening
you have possible signs of sepsis, such as breathing very fast, being confused or having slurred speech, blue, pale or blotchy skin, a rash that does not fade when you press it, or a very high or very low temperature. Being pregnant makes sepsis more likely
you do not feel you can keep yourself safe (a mental health emergency is as serious as a physical one)

Always call 999 if you think it's a life-threatening emergency, no matter what stage of pregnancy you are.

Contact your GP or early pregnancy unit right away if:

you have light bleeding or spotting, even if you have no pain or only mild tummy pain
you have vaginal discharge that's unusual for you, or fluid leaking from your vagina
you have tummy pain low down on one side, or pain in the tip of your shoulder (where your shoulder ends and your arm begins). Symptoms of an ectopic pregnancy usually start between 4 and 12 weeks, so get advice right away, even if you have not had a positive pregnancy test yet
you have stomach pain that is severe or does not go away after you've rested for 30 to 60 minutes, or stomach pain with regular cramping or tightenings, lower back pain, or pain when you pee
your breathlessness comes on suddenly, you're out of breath when you're lying down, or you're breathless and also have chest pain or feel dizzy
you have pain or swelling in one leg, usually in the calf or thigh. This could be a blood clot (deep vein thrombosis, or DVT), which is more likely when you're pregnant
you have a high temperature (38C or above), or you feel hotter than usual or shivery. A high temperature can be a sign of an infection, so it's important to get it checked

If you cannot reach them, or you're not sure what to do, call NHS 111.

Speak to your GP or midwife if:

you're worried about your mental health, or stress, anxiety or low mood are affecting your day-to-day life. If you need urgent help for your mental health, call 111 and select the mental health option, or ask for an urgent GP appointment

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.

Sources & further reading

General information, not individual medical advice. Speak to your GP, midwife or health visitor about your baby’s needs.