Week 5 is the week the heart starts beating and the nervous system begins to form. It is also, for most women, the week symptoms arrive — and the week that makes folic acid matter more than at any other point in the pregnancy.
The embryo is about 2 mm long, roughly the size of a sesame seed, and is now building the two structures everything else depends on.
The short answer: the neural tube — the future brain and spinal cord — folds and begins closing this week, finishing by about week 6, which is exactly why folate is critical now. The heart tube makes its first irregular contractions, though it is usually too early to see on a scan. Tiredness, sore breasts, frequent urination and the start of nausea are all typical.
Your baby this week
Neurulation is the headline event. A groove forms along the embryo's back and folds inward, its edges meeting and fusing into the neural tube. The top end becomes the brain, the rest becomes the spinal cord. Closure runs from the middle outwards in both directions and finishes around week 6 — and if it does not close completely, conditions such as spina bifida result. This is the entire reason folic acid is recommended before conception rather than after a positive test.
At the same time the heart tube forms, bends into an S-shape and begins contracting. These first beats are irregular and the heart has no chambers yet, but circulation has started. Blocks of tissue called somites appear either side of the neural tube; they will become the vertebrae, ribs and the muscles of the back.
The embryo is C-shaped with a visible tail that disappears in the coming weeks. Buds that will become arms and legs are not there yet — they appear next week.
Your body this week
Oestrogen and progesterone are both high, and the combination explains most of how you feel. Progesterone is sedating, which is why the tiredness of early pregnancy is often described as unlike ordinary tiredness. The same hormones drive emotional volatility — that is chemistry rather than character, and it settles.
The uterus becomes softer and rounder, a change your doctor can feel on examination. It has also started pressing on the bladder, which is why you may already be passing urine more often. That is normal; burning, urgency with pain, or a fever is not, and needs treating rather than tolerating — urinary infections are more common in pregnancy and more likely to spread to the kidneys.
Nausea often begins around now. It is worst on an empty stomach, which is why the standard advice is small amounts often rather than three meals. Our morning sickness guide covers what actually helps.
Tests and appointments
Nothing routine is scheduled at 5 weeks. An early scan is sometimes arranged — after a previous ectopic pregnancy or recurrent miscarriage, with IVF, or if there is pain or bleeding — but a scan this early often shows only a gestational sac. That is expected, not a bad sign, and a repeat a week or two later is how the picture is confirmed.
✅ Worth doing this week
- Do not miss folic acid. This is the week it does its work — the neural tube closes over the next several days
- Eat small and often if nausea has started; an empty stomach makes it worse
- Add iodine and omega-3 sources — dairy, eggs, white fish and low-mercury oily fish support brain and thyroid development
- Rest without guilt. First-trimester fatigue is hormonal, not a lack of discipline
- Report any burning on passing urine rather than waiting — infections need treating in pregnancy
When to call your doctor
Call the same day if: you have bleeding heavier than light spotting, severe or one-sided abdominal pain, shoulder-tip pain, dizziness or fainting, a fever, or vomiting so persistent that you cannot keep fluids down for 24 hours. Relentless vomiting can become hyperemesis gravidarum, which is treatable and should not be endured.
FAQ — 5 weeks pregnant
Is there a heartbeat at 5 weeks?
The heart tube starts contracting around now — often described as day 22 after fertilisation — but it is not usually visible on a scan yet. A transvaginal ultrasound may show a heartbeat from about 5.5 to 6 weeks, and not seeing one at 5 weeks means almost nothing. Dating is frequently a few days out at this stage, which is why a repeat scan a week later is standard rather than alarming.
How big is the baby at 5 weeks?
About 2 mm crown to rump — roughly a sesame seed. The embryo is C-shaped with a visible tail, and the structures forming now are the neural tube, the heart tube and the first somites, the blocks that become the spine and muscles.
Why does folic acid matter so much at 5 weeks?
Because the neural tube — the structure that becomes the brain and spinal cord — is closing right now and completes by about week 6. Folate is required for that closure. Taking it after week 6 does not undo an incomplete closure, which is why the advice is always to start before conception. If you have only just found out and only just started, take it now anyway: it still supports everything that follows.
Is cramping normal at 5 weeks?
Mild cramping is common and usually reflects the uterus changing shape and its supporting ligaments stretching. What is not routine is cramping that is severe, one-sided, comes with bleeding, or is accompanied by dizziness or shoulder-tip pain — those need same-day assessment because early pregnancy pain cannot be sorted into safe and unsafe by feel alone.
Week 5 at a glance
At 5 weeks the embryo is about 2 mm and doing two decisive things: folding its neural tube, which becomes the brain and spinal cord and closes by roughly week 6, and forming a heart tube that begins its first irregular contractions. The neural tube timing is the whole argument for folic acid before conception. A scan this early often shows only a gestational sac, and no visible heartbeat at 5 weeks is not a bad sign. Tiredness, tender breasts, frequent urination and the beginning of nausea are all typical, and feeling little is also within normal.
Inside Baby Novum: supplement reminders are tied to the week they matter rather than a generic daily nudge, and the symptom journal records nausea severity over time — which is the information a doctor needs if it escalates towards hyperemesis.