# 4 Weeks Pregnant: Baby Size, Symptoms & What to Expect | Clever Mum > 4 weeks pregnant — your baby is the size of a poppy seed. What's developing, what you might feel, and what to focus on this week. * [ Home ](/) * › [ Pregnancy ](/pregnancy/) * › [ Week by week ](/pregnancy/week/) * › Week 4 First trimester # 4 weeks pregnant Your baby is about the size of a poppy seed [ CP ](/authors/parenting-desk/) By [ Clever Mum Parenting ](/authors/parenting-desk/) · Parenting guides Published 11 July 2026 Quick answer — What happens at 4 weeks pregnant? At 4 weeks pregnant, your baby is about the size of a poppy seed. The fertilised egg has implanted in the womb lining and is now an embryo. Who should skip this Skip this page if you need personal medical advice — it is general NHS-aligned information, not a substitute for your midwife. If your baby's movements slow or change, call your maternity unit, not a website. ## How big is the baby at 4 weeks pregnant? At 4 weeks pregnant, your baby is about the size of a poppy seed. The fertilised egg has implanted in the womb lining and is now an embryo. The cells are organising into the layers that will become your baby's nervous system, organs and skin — and the placenta is beginning to form. These are typical averages — your midwife's measurements are the ones that matter for your pregnancy. ## What might you feel at 4 weeks? Most people at 4 weeks notice some mix of the symptoms below — and plenty notice none of them. This is often the week a missed period prompts a positive test. Many people feel nothing yet; others notice early tender breasts, mild cramping or extra tiredness. Implantation spotting can happen around now. None of this is a diagnosis; contact your midwife if something worries you. ### Common symptoms this week * Missed period * Mild cramping or implantation spotting * Tender, heavier breasts * Tiredness Symptom lists are typical, never exhaustive — and having none of them is normal too. ## What should you do at 4 weeks pregnant? The useful job this week is practical, not medical. Start (or continue) a 400mcg folic acid supplement and 10mcg vitamin D daily, and book in with your GP or self-refer to your local midwife service. Pair it with the NHS week-by-week guide if you want a second description of the same stage. ### 🏥 NHS Care & Clinical Appointments * ✓ Self-referral to local NHS maternity unit or GP booking ### Week 4 checklist * Take a pregnancy test from the first day of your missed period * Start 400mcg folic acid and 10mcg vitamin D daily * Book your first midwife (booking) appointment — self-referral works in most areas ## What else is useful at 4 weeks? Use the week page for size and symptoms; use the guides below for the decisions that actually sit in this window — pay, packing, car seats, labour signs. * [Due date calculator](/tools/due-date-calculator/) * [Trimester to-do list](/pregnancy/pregnancy-to-do-list-by-trimester/) ## 4 weeks pregnant: common questions Can I take a pregnancy test at 4 weeks? Yes — 4 weeks pregnant corresponds to the first day of a missed period, which is exactly when home tests are designed to be reliable. Use first-morning urine, and retest in three days if it's negative but your period still hasn't arrived. How big is my baby at 4 weeks pregnant? At 4 weeks, your baby is approximately the size of a poppy seed. These are approximate averages — every baby grows at its own pace. What's happening at 4 weeks pregnant? The fertilised egg has implanted in the womb lining and is now an embryo. The cells are organising into the layers that will become your baby's nervous system, organs and skin — and the placenta is beginning to form. For you: this is often the week a missed period prompts a positive test. many people feel nothing yet; others notice early tender breasts, mild cramping or extra tiredness. implantation spotting can happen around now. Which trimester is 4 weeks? 4 weeks pregnant is in the first trimester. The first trimester runs to the end of week 12, the second from weeks 13 to 26, and the third from week 27 to birth. ## Sources & further reading * [ You and your baby at 4 to 40 weeks pregnant ](https://www.nhs.uk/pregnancy/week-by-week/) — NHS * [ Your baby's movements ](https://www.nhs.uk/pregnancy/keeping-well/your-babys-movements/) — NHS [All weeks](/pregnancy/week/) [5 weeks →](/pregnancy/week/5/) This is general information, not medical advice — every pregnancy is different. For anything that worries you, and for all your antenatal care, speak to your midwife or GP. If your baby's movements ever slow down or change, contact your maternity unit immediately, day or night. Reviewed August 2026. ## Week 4 at a glance Trimester First trimester Baby size poppy seed Weeks to go 36 (to 40) Work out your dates Not sure how many weeks you are? Our [due date calculator](/tools/due-date-calculator/) maps your weeks and trimester dates from your last period.