First Trimester Timeline: What's Happening Week by Week

The first trimester runs from week 1 to the end of week 13. Because gestational age is counted from the first day of your last menstrual period rather than from conception, weeks 1 and 2 precede conception entirely — you are counted as two weeks pregnant at the moment it happens.

This convention catches nearly everyone out. It also means that by the time a period is missed and a test is positive, you are typically already around four weeks pregnant. The Due Date Calculator will convert dates into gestational age.

What follows is a general outline. Every pregnancy differs, and this is information rather than medical advice — your maternity team is the right source for anything specific to you.

Weeks 1 and 2 cover your period and the run-up to ovulation. Conception occurs around the end of week 2.

In week 3 the fertilised egg divides as it travels down the fallopian tube toward the uterus. During week 4 it implants in the uterine lining, and the cells that will become the placenta begin producing hCG — the hormone pregnancy tests detect. Some people notice light implantation spotting around this point.

By the end of week 4, a sensitive test taken around the time of a missed period will usually be positive.

This is the period of most intense organ formation, and it is when the embryo is most vulnerable to disruption.

The neural tube — which becomes the brain and spinal cord — forms and closes during weeks 5 and 6, which is why folic acid supplementation is recommended before conception and through the first trimester. The heart begins beating around week 6 and is usually detectable on ultrasound by then. Limb buds appear, and facial features begin to form.

By the end of week 8 the embryo is around 1.6 cm long, all major organ systems have begun forming, and it is formally reclassified as a fetus.

Development shifts from forming structures to refining them. Fingers and toes separate, external genitalia begin to differentiate, and the kidneys start producing urine. Reflexive movement begins, though it is far too slight to feel.

The placenta takes over hormone production from the corpus luteum around weeks 10 to 12, which is one reason nausea often eases toward the end of the trimester.

By the end of week 13 the fetus is roughly 7 to 8 cm long. The risk of miscarriage falls markedly after this point, which is why many people wait until then to share the news.

First-trimester symptoms are driven largely by rapidly rising hCG and progesterone. Their intensity varies enormously, and having few symptoms is not a cause for concern.

Most maternity services schedule a booking appointment between weeks 8 and 12. It is usually the longest appointment of the pregnancy: full medical history, blood tests, blood pressure, and a discussion of screening options.

A dating scan is typically offered between 11 and 14 weeks, and it is the most accurate dating you will get — measuring crown-rump length at this stage predicts gestational age within about five to seven days.

Combined screening for chromosomal conditions is usually offered around the same window, generally pairing a nuchal translucency measurement with blood markers. Non-invasive prenatal testing from maternal blood is available from around 10 weeks in many places. Screening is optional, and what is offered varies considerably by country.

Some symptoms warrant prompt contact with your maternity team or doctor rather than waiting for the next appointment.

Most first-trimester information avoids this topic, which leaves people who experience it feeling both blindsided and unusually unlucky. They are neither.

Around 10 to 20% of clinically recognised pregnancies end in miscarriage, the large majority in the first trimester, and the true rate including very early losses is higher still. The most common cause by a wide margin is a chromosomal abnormality arising at conception — a random event, not something caused by working, exercising, stress, or anything the pregnant person did.

Risk falls substantially as the first trimester progresses, and after a scan showing a heartbeat at around 8 weeks it drops considerably further. Anyone experiencing bleeding or cramping should contact their maternity team, who can arrange assessment. Support is available, and early loss is far more common than the silence around it suggests.

Most people continue working through the first trimester, often before telling anyone. Employment protections and the point at which you must notify an employer vary by country, but in most places you are not obliged to disclose early — and where the work involves specific hazards, a risk assessment is typically required once you do.

Travel is generally considered safe in an uncomplicated first trimester. Airlines rarely restrict flying until much later in pregnancy, though the increased clotting risk means moving around the cabin and staying hydrated matter more than usual. Some destinations carry infection risks worth checking before booking.

The questions that come up most — caffeine limits, exercise intensity, which foods to avoid, whether a particular medication is safe — all have answers that differ somewhat between countries and health services. Use the guidance your own maternity service publishes rather than general internet advice, and ask a pharmacist about anything you take regularly.

The general guidance is consistent across most health services: take folic acid, avoid alcohol and smoking, follow local advice on foods to avoid, and check any medication — including over-the-counter remedies and supplements — with a pharmacist or doctor.

Physical activity is generally encouraged in uncomplicated pregnancies, and the first trimester rarely requires modifying what you already do. Fatigue, however, is real and worth respecting rather than pushing through.

Weight gain in this trimester is typically minimal, commonly 0.5 to 2 kg in total, and many people gain nothing. The Pregnancy Weight Gain Calculator compares your gain against the expected trajectory for your week, which is a more useful check than any single number.