First trimester: tests and appointments, weeks 1 to 13

If you are pregnant, or might be, and something worries you, do not wait for a scheduled appointment. The NHS treats bleeding in pregnancy as an emergency when it comes with severe abdominal pain, pain in the shoulder, or feeling sick, faint or dizzy, and when it is heavy enough to soak a period pad soon after you put it on (NHS, "Vaginal bleeding in pregnancy", 2026). In the UK, call the emergency number or go to A&E, the hospital emergency department; in the US, call the emergency number or go to the nearest emergency room. The box above shows the emergency and urgent numbers for where you are, and get help now lists the other symptoms that need checking straight away.

Weeks 1 and 2: the count begins before you are pregnant

Pregnancy is counted from the first day of the last period, so the first two weeks come before conception (ACOG, "How Your Fetus Grows During Pregnancy", 2024). The March of Dimes says conception usually happens about 2 weeks after the last period starts (March of Dimes, 2023). Estimated gestational age explains the counting.

Weeks 3 and 4: the fertilized egg attaches to the uterus

The March of Dimes places implantation in weeks 3 to 4, when the fertilized egg reaches the uterus and attaches to its lining (March of Dimes, 2023). Wilcox, Baird and Weinberg followed 221 women trying to conceive and found that the pregnancy hormone first appeared in urine 6 to 12 days after ovulation in pregnancies lasting at least six weeks, with implantation on day 8, 9 or 10 in 84 in 100 of them (New England Journal of Medicine, 1999). When does implantation occur sets out that timing in full, and implantation bleeding covers the bleeding that can come with it.

Weeks 4 to 6: a missed period and the first symptoms

The NHS says symptoms and early signs usually start when you are around 4 to 6 weeks pregnant, and that not everyone has all of them (NHS, "Signs and symptoms that might mean you're pregnant", 2026). It lists a missed period; a light bleed or spotting around the time the period is due; feeling or being sick; extreme tiredness; sore breasts; needing to pass urine more often; constipation; more vaginal discharge; a metallic taste; sensitivity to smells; cravings or loss of interest in some foods; and mood changes (NHS, "Signs and symptoms that might mean you're pregnant", 2026). The US Office on Women's Health gives a similar list for the first trimester (Office on Women's Health, 2025). Early pregnancy symptoms sets the NHS list beside Mayo Clinic's, and when does morning sickness start covers the timing of nausea. This guide cannot say what any of these means for you.

The NHS says vaginal bleeding, stomach pain and pelvic pain may be a sign of something more serious, and advises getting medical advice if you are worried about your health or your baby's at any time (NHS, "Signs and symptoms that might mean you're pregnant", 2026). How soon can you take a pregnancy test covers when a test can show a result. Spotting during pregnancy and cramping in early pregnancy set out when bleeding or pain needs help.

Weeks 1 to 12: what ACOG says is developing

ACOG's patient guidance says that in weeks 1 to 8 the brain and spine begin to form, cardiac tissue starts to develop and the muscles of the eyes, nose and mouth form. In weeks 9 to 12 the kidneys begin making urine, the pancreas starts making insulin and fingernails form (ACOG, "How Your Fetus Grows During Pregnancy", 2024).

Weeks 8 to 12: the first appointment

In the UK, the NHS describes the first midwife appointment, sometimes called the booking appointment, at 8 to 12 weeks, lasting around an hour (NHS, "Antenatal (pregnancy) care and appointments", 2026). It covers questions about your health and the date of your last period, information about screening tests, a blood pressure check, a urine test and an offered blood test to check your blood group. NICE guideline NG201 advises offering the booking appointment by 10 weeks and 0 days, and within 2 weeks if possible for someone who first contacts services after 9 weeks and 0 days (NICE, 2021). You can refer yourself for NHS pregnancy care as soon as you know you are pregnant (NHS, "Antenatal (pregnancy) care and appointments", 2026).

In the US, ACOG's patient guidance on prenatal care lists what to bring to a first visit, including the first day of your last period to help establish your due date, a list of medications, vitamins and supplements, and your health and pregnancy history (ACOG, "Prenatal Care", 2025). The same page says it is best to start prenatal care in the first trimester, that before 10 weeks after the last period is ideal, and that the timing can be earlier or later depending on when you find out you are pregnant and how quickly you can find an ob-gyn (ACOG, 2025). Ask your own provider when they want to see you. Routine tests early in pregnancy include a complete blood count, blood type and Rh factor, urinalysis, urine culture, and tests for rubella, hepatitis B and C, HIV, other sexually transmitted infections and tuberculosis (ACOG, "Routine Tests During Pregnancy", 2024).

Weeks 11 to 14: the dating scan

NICE advises offering a scan between 11 weeks and 2 days and 14 weeks and 1 day to determine gestational age and detect a multiple pregnancy (NICE, 2021), and the NHS describes it as the 12-week scan, offered at around 10 to 14 weeks (NHS, "12-week scan", 2023). It can be combined with screening for Down's, Edwards' and Patau's syndromes if you choose it. Due date based on ultrasound explains how the scan can change the due date.

Where the first trimester ends depends on the source

ACOG defines the first trimester as the first day of the last period to 13 weeks and 6 days (ACOG, "How Your Fetus Grows During Pregnancy", 2024). The NHS labels weeks 4 to 12 as the first trimester (NHS, "Week-by-week guide to pregnancy", 2023) and the Office on Women's Health weeks 1 to 12 (2025). The hub for pregnancy trimesters sets the three side by side, and the guide to the second trimester picks up where this one ends. Pregnancy weeks to months converts the weeks into months. The due date calculator and the pregnancy week calculator work out where you are on this timeline from your dates.

This guide is information, not a diagnosis or a recommendation of treatment, and it is no substitute for your own clinician. See the medical disclaimer.