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 contains the fastest stretch of fetal development and the highest miscarriage risk. Most people feel the most tired during these weeks and may experience nausea between weeks 6 and 14. By week 13 the risk of pregnancy loss drops sharply and most external organs are formed.

Weeks 1 to 4: fertilization and implantation

Because due dates are measured from LMP, 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 between days 6 and 10 after fertilization, usually during week 3 or early week 4. Around this time hCG production starts, which is what home pregnancy tests detect. Most people get a positive test between weeks 4 and 5. To find your personal week count from either LMP or conception, use our Due Date Calculator.

Weeks 5 to 8: the embryonic stage

Between weeks 5 and 8 the embryo develops the neural tube, the beginnings of the heart, and the buds that become arms and legs. A heartbeat is usually detectable by transvaginal ultrasound around week 6 to 7. By week 8, all major organs have started to form, and the embryo is about 1.5 cm long. This is the period of highest vulnerability to teratogens, which is why OBs review medications, alcohol, and smoking early. Folic acid supplementation (typically 400 mcg per day) is strongly recommended from pre-conception through at least week 12 to reduce neural-tube defect risk.

Weeks 9 to 13: fetal stage begins

From week 9 the embryo is technically a fetus. Fingers and toes separate, external genitalia start to differentiate (though not visible on ultrasound yet), and movement begins, too small to feel. The first-trimester combined screening test is usually performed between weeks 11 and 13 and 6 days. It combines the nuchal translucency ultrasound with maternal blood markers. NIPT can be done from week 10 and screens for common chromosomal conditions with very low false-positive rates. By week 13 the fetus is about 7 cm long and miscarriage risk drops sharply.

Common symptoms and why they happen

Fatigue, nausea, breast tenderness, frequent urination, and mood shifts are the most common first-trimester experiences, and all are driven by the hormonal changes that support implantation and placenta development. Nausea typically starts around week 6, peaks between weeks 8 and 10, and resolves for most people by week 14. Persistent vomiting that prevents fluid intake is called hyperemesis gravidarum and needs clinical care. Spotting is not uncommon and is not automatically a warning sign, but heavy bleeding or severe one-sided pain should be reviewed urgently.

Key tests during the first trimester

Three appointments anchor most first-trimester care. The first visit, usually between weeks 8 and 10, includes confirming the pregnancy, a dating ultrasound, a full history, and baseline blood work (blood type, antibody screen, CBC, infections screen). The dating ultrasound between weeks 8 and 14 measures CRL and can override LMP if the dates disagree by more than 5 to 7 days, per ACOG Committee Opinion 700. The first-trimester screening between weeks 11 and 13+6 combines nuchal translucency with blood markers, and optional NIPT from week 10 can be added for higher-precision screening.

When to call your OB

Five signals warrant a call rather than waiting for the next visit. Heavy vaginal bleeding. Severe or one-sided pelvic pain (a possible ectopic sign). Persistent vomiting without fluid retention. High fever (over 38.5 C or 101 F). 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 a 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 and is the definitive look at fetal structure, including sex determination for most families. 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 usually begins around week 6 and peaks between weeks 8 and 10. Around 70 percent of pregnancies are symptom-free by week 14. Persistent vomiting that prevents fluids needs clinical care.

When is the dating ultrasound?

Usually between weeks 8 and 14. A first-trimester CRL measurement can re-date the pregnancy if it disagrees with LMP by more than 5 to 7 days.

When can NIPT be done?

NIPT is available from about 10 weeks. It screens for common chromosomal conditions and reports fetal sex with over 99 percent accuracy in most published cohorts.

What is the miscarriage risk in the first trimester?

Overall first-trimester loss risk is about 10 to 15 percent for confirmed pregnancies. By 10 weeks with a healthy heartbeat, the risk drops to under 2 percent.

When should you announce a pregnancy?

Many people wait until after the 12-week dating ultrasound because the loss risk drops sharply by then. There is no medical rule, only the statistical rationale for the 12-week threshold.

Bottom line

The first trimester is the period of the fastest development and the highest loss risk, and it is also the stretch most people feel worst during. By week 13 the baseline shifts: energy returns, nausea fades for most, and the loss risk drops sharply. 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.