5 weeks pregnant: development, the heartbeat and first signs
5 weeks pregnant is around the time, the NHS says, that many women realise they are pregnant, with an embryo of about 2 mm whose brain and spinal cord are forming rapidly (NHS, 2026). This page sets out what the NHS, ACOG and the Office on Women’s Health say about week 5.
Last updated
Medically reviewed by Dr Mary Davidson, Reproductive endocrinologist, checked on
Seek help now if you have any of these symptoms
Bleeding and cramping are common in early pregnancy and often do not mean miscarriage. The symptoms below, however, need to be assessed straight away.
- Soaking two or more sanitary pads per hour, or passing large clots
- Severe lower abdominal pain, especially on one side
- Pain at the tip of the shoulder
- Dizziness, fainting, or collapse
- Signs of shock — pale, clammy, rapid heartbeat, breathlessness
- Pain on opening bowels, or bladder or bowel changes
- Feeling very unwell, or feverish
Showing routes for United States.
How the sources describe development around 5 weeks
- Size and shape, NHS
- The NHS week 5 page in its week-by-week guide (NHS, 2026) gives the embryo as around 2 mm long and compares it to a sesame seed. It says the face is starting to take shape, with a tiny nose and little eyes that stay closed until around 28 weeks, and that the brain and spinal cord are forming rapidly.
- Heart, blood vessels and placenta, NHS
- The NHS says the tiny heart is starting to form and will beat for the first time around now. It says the embryo already has some blood vessels of its own, a string of which will make up the umbilical cord, and that the placenta, which is being created now, will give the baby nutrients and oxygen and remove waste products (NHS, 2026).
- Development, Office on Women’s Health
- At four to five weeks the Office on Women’s Health page on the stages of pregnancy (2025) says the brain and spinal cord have begun to form, the heart begins to form, arm and leg buds appear, and the embryo is one-twenty-fifth of an inch long.
Weeks are counted from the first day of the last menstrual period, so about two weeks of the count come before conception (ACOG, How Your Fetus Grows During Pregnancy, 2024). This page covers 5 weeks 0 days to 5 weeks 6 days.
A normal pregnancy lasts about 40 weeks from the first day of the last menstrual period, so at 5 weeks about 35 weeks remain to the estimated due date (ACOG, How Your Fetus Grows During Pregnancy, 2024). Your own due date depends on your last period or a scan, not on the week alone.
Your body at 5 weeks, as the NHS describes it
The NHS says you might notice that your period is late and feel a bit under the weather, and that not everyone has regular cycles, so you may not realise a period is late. It says extreme tiredness is common in the first trimester and repeats its list of other early signs, including sore breasts, nausea, mood swings, new likes and dislikes in food, a heightened sense of smell and needing to pee more often (NHS, 2026). Early pregnancy symptoms lists them all.
On bleeding, the NHS says you might notice some light bleeding and think it is your period, but that it can also be a sign of implantation bleeding, when an embryo attaches to the lining of the womb. In the same list it says to see your doctor if you get bleeding in pregnancy (NHS, 2026). This page cannot tell you what any bleeding means, and the box above says when to get help. Spotting during pregnancy sets out how the NHS describes spotting and when it needs urgent help.
Testing and feelings
The NHS says pregnancy tests are sensitive to changes in the urine from week 3 or 4 onwards, so week 5 is a good time to test if you are wondering (NHS, 2026). How soon can you take a pregnancy test compares the UK and US advice on timing. It also says that finding out you are pregnant can be exciting and that it is normal to have worries too: around 1 in 5 mums feel anxious or stressed during pregnancy, and it asks people to talk to their midwife or doctor rather than keep worries to themselves (NHS, 2026).
ACOG describes weeks 1 to 8 as a period in which the muscles of the eyes, nose and mouth form and webbed fingers and toes poke out from the developing hands and feet (ACOG, How Your Fetus Grows During Pregnancy, 2024).
Work out which week you are in
The Pregnancy week calculator takes your last period, conception date, scan or IVF transfer and shows how many weeks and days pregnant you are today.
Sources
- NHS Best Start in Life. Week 5 of pregnancy (week-by-week guide to pregnancy). nhs.uk, 2026.
- American College of Obstetricians and Gynecologists. How Your Fetus Grows During Pregnancy (FAQ156). ACOG, 2024.
- Office on Women's Health, US Department of Health and Human Services. Stages of pregnancy. womenshealth.gov, 2025.
Frequently asked questions
When does the heart start to beat?
The NHS says the heart is starting to form at 5 weeks and will beat for the first time around now (NHS, 2026). ACOG lists cardiac tissue starting to develop somewhere in weeks 1 to 8 (ACOG, 2024), and the Office on Women’s Health says the heart begins to form at four to five weeks and beats with a regular rhythm at eight weeks (Office on Women’s Health, 2025).
Is light bleeding at 5 weeks implantation bleeding?
The NHS says light bleeding at this stage may be taken for a period or may be a sign of implantation bleeding, and it also says to see your doctor if you get bleeding in pregnancy (NHS, 2026). This page cannot say what any bleeding is. Use the box above for the symptoms that need urgent help.