First Trimester Week by Week: What to Expect Through Week 13

The first trimester covers weeks 1 through 13 of pregnancy, measured from the first day of your last period. It includes a fast stretch of fetal development and carries a higher loss risk than later pregnancy.^[14] Many people feel their most tired during these weeks, and nausea or vomiting often start before week 9 and ease by about week 14 for most pregnancies.^[1]

Weeks 1 to 4: fertilization and implantation

Because due dates are measured from the last period, weeks 1 and 2 are technically before conception. Ovulation and fertilization happen around the end of week 2. The fertilized egg becomes a blastocyst and implants into the uterine lining during week 3 or early week 4.^[11] hCG production starts around then, which is the hormone home pregnancy tests detect.^[2] A urine test usually picks up hCG from the first day of a missed period;^[12] a blood test can detect it earlier.^[2] To find your personal week count from either the last period or a known conception date, use our Due Date Calculator.

Weeks 5 to 8: the embryonic stage

Between weeks 5 and 8 the embryo develops the neural tube, the early heart structures, and the buds that become arms and legs. A heartbeat is usually detectable by transvaginal ultrasound around week 6 to 7. By week 8, major structures have started to form and the embryo is still very small.^[11] This is the period of highest vulnerability to teratogens, which is why clinicians review medications, alcohol, and smoking early. Taking 400 mcg of folic acid daily before pregnancy and during early pregnancy helps prevent neural-tube defects.^[3]

Weeks 9 to 13: fetal stage begins

During weeks 9 to 13, essential organs continue developing and the transition from embryo to fetus happens by the end of week 10. Fingers, toes, nails, and genital structures continue forming across this stretch.^[11] The first-trimester combined screening is usually performed between weeks 11 and 13 plus 6 days, and pairs the nuchal translucency ultrasound with maternal blood markers.^[4] Cell-free DNA screening (NIPT) can be done from about 10 weeks; it is the most sensitive screening test for common fetal aneuploidies, but it remains a screening test rather than a diagnostic test.^[4]

Find your week

For a closer look at one specific week, use the dedicated weekly guides below. Each guide gives a closer look at that week and includes a safety section.

• 4 weeks pregnant
• 5 weeks pregnant
• 6 weeks pregnant
• 7 weeks pregnant
• 8 weeks pregnant
• 9 weeks pregnant
• 10 weeks pregnant
• 11 weeks pregnant
• 12 weeks pregnant
• 13 weeks pregnant
• Pregnancy Week by Week hub

Common symptoms and why they happen

Nausea and vomiting of pregnancy typically begin before week 9 and ease by about week 14 for most pregnancies.^[1] Many people also notice fatigue, breast tenderness, frequent urination, or mood shifts in early pregnancy; if any symptom feels severe, sudden, or different from your baseline, use the safety guidance below. Persistent vomiting that prevents fluid intake is called hyperemesis gravidarum and needs clinical care.^[1] Light spotting can happen in early pregnancy, but heavy bleeding should be reviewed urgently.^[5] Severe or one-sided pelvic pain can be a sign of ectopic pregnancy and should be reviewed promptly.^[13]

Key tests during the first trimester

Three appointments anchor most first-trimester care. The first visit, usually in the first trimester, includes a full history and baseline blood work. UK booking tests commonly include blood group and rhesus D status, full blood count, screening for sickle cell and thalassaemia, HIV, syphilis, hepatitis B, and urine testing.^[6][7]

The dating ultrasound in UK care is usually offered at 11 to 14 weeks.^[7] In US dating guidance, first-trimester crown-rump length is the most accurate ultrasound measure for confirming or revising gestational age. If the ultrasound-based date disagrees with the last period by more than 5 to 7 days in the first trimester, the ultrasound date takes over.^[8] First-trimester combined screening between weeks 11 and 13 plus 6 days pairs nuchal translucency with blood markers; NIPT from about 10 weeks can be added.^[4]

When to call a clinician

Several signals warrant a call rather than waiting for the next visit. Heavy vaginal bleeding.^[5] Severe or one-sided pelvic pain, which can be a sign of ectopic pregnancy.^[13] Persistent vomiting without fluid retention.^[1] A fever of 100.4 °F (38 °C) or higher.^[9] Any sudden severe symptoms that feel different from your baseline. None of these automatically mean something is wrong, but all are reasons for same-day review rather than wait-and-see.

Planning for the second trimester

As the first trimester ends, energy usually returns and nausea fades for most people. The anatomy scan is scheduled for weeks 18 to 22; if the fetus is in a good position, it may be possible to learn the sex at that scan.^[10] If you are thinking about timing announcements, planning parental leave, or looking at the full 40-week map, our Due Date Calculator shows remaining weeks and trimester breakdown at a glance, and our Conception Calculator is useful if you want to trace back to a known conception event for IVF or tracked cycles.

FAQ

When does morning sickness start?

Nausea and vomiting of pregnancy usually begin before week 9 and ease by about week 14 for most pregnancies.^[1] If vomiting prevents you from keeping fluids down, contact a clinician the same day.

When is the dating ultrasound?

In UK care, the dating scan is usually offered at 11 to 14 weeks.^[7] A first-trimester crown-rump length measurement can re-date the pregnancy if it disagrees with the last period by more than 5 to 7 days.^[8]

When can NIPT be done?

NIPT can be done from about 10 weeks. It is the most sensitive screening test for common fetal aneuploidies, but it remains a screening test, not a diagnostic test.^[4]

What is the miscarriage risk in the first trimester?

First-trimester loss risk is higher than later in pregnancy and decreases as the pregnancy progresses.^[14] The exact risk depends on gestational age and personal history. Discuss your individual risk with your clinician.

When should you announce a pregnancy?

There is no medical rule for when to announce a pregnancy. People choose different times based on privacy, support needs, and personal comfort.

Bottom line

The first trimester is a period of fast development and higher loss risk than later pregnancy. Exact risk changes with gestational age and personal history. For a personalized week count and trimester breakdown, use our Due Date Calculator. If you are curious about how due date and conception date differ, see our companion article.