The first trimester runs from the first day of your last menstrual period through the end of week 13 — thirteen weeks in which more developmental change happens than at any other point in a human life. Most of those weeks pass before you even take a positive pregnancy test. This guide walks through what is actually happening week by week, which symptoms are normal (and which are not), and what to expect from the medical side: first prenatal visit, blood work, ultrasound dating, and the optional screening tests available in this window.
Weeks 1–4: before you know
In obstetric time, weeks 1 and 2 of pregnancy predate fertilization. Week 1 is your menstrual period; week 2 is the follicular growth phase leading to ovulation around day 14 of a 28-day cycle. Fertilization happens around the end of week 2.
By week 3, the fertilized egg (zygote) has begun dividing and traveling down the fallopian tube toward the uterus. It arrives as a blastocyst around day 5 post-fertilization and begins implanting into the endometrium between days 6 and 12 post-fertilization (roughly days 20–26 of a 28-day cycle).
Week 4 is when most home pregnancy tests become reliably positive. The blastocyst has implanted, the embryonic disk is forming, and the placenta’s precursor — the trophoblast — is producing hCG at levels detectable in urine. By the end of week 4, the primitive streak is forming, and the three embryonic germ layers (ectoderm, mesoderm, endoderm) that will become every structure in the body are being laid down.
At 4 weeks, the embryo is approximately 2 mm — smaller than a grain of rice.
What you might feel: Nothing yet, or very light cramping and spotting (implantation). Some women notice breast tenderness and fatigue in week 4 as hCG rises.
Weeks 5–8: the embryonic period
This eight-day range is the most developmentally critical stretch of the entire pregnancy. All major organ systems are initiated during the embryonic period.
Week 5
The neural tube — which will become the brain and spinal cord — begins forming and closes by the end of week 6. This is why adequate folate before and in early pregnancy matters: neural tube defects (NTDs) arise from failures in neural tube closure, most of which occur before week 8, often before the pregnancy is confirmed. ACOG recommends 400 micrograms of folic acid daily for women of reproductive age, and 600 micrograms during pregnancy (ACOG Committee Opinion 313).
The heart begins forming as a simple tube and begins beating (electrical activity) around day 22 of embryonic development, which is approximately 5 weeks 3 days gestational age. The heartbeat becomes detectable by transvaginal ultrasound typically between 6 and 7 weeks.
Week 6
The embryo is 4–5 mm. Limb buds appear. The heart has four chambers forming. The eyes, ears, and primitive gut are differentiating. The yolk sac — visible on early ultrasound — is providing nutrition before the placenta takes over.
Week 7
The embryo is 10–13 mm. The head is disproportionately large relative to the body — the brain is growing faster than anything else. Fingers are beginning to form from paddle-shaped hand plates. The kidneys are taking shape.
Week 8
The embryo graduates from embryo to fetus at the end of week 8 (by convention, though “embryo” and “fetus” are sometimes used interchangeably in the first trimester). It is 16–18 mm. All major organs are represented in some form. The tail, present in earlier weeks, disappears. Facial features are more defined. The external genitalia are present but not yet sexually differentiated.
What you might feel, weeks 5–8: This is when first-trimester symptoms typically peak.
- Nausea and vomiting (morning sickness, which can occur at any time): affects 70–80% of pregnant women. Driven primarily by rapidly rising hCG levels. Typically peaks around weeks 8–10.
- Breast tenderness and enlargement: one of the earliest and most consistent symptoms.
- Fatigue: often severe. The body is producing a placenta and growing exponentially.
- Frequent urination: increasing blood volume and hCG effects on the kidneys.
- Food aversions and heightened smell sensitivity: common from week 5 onward.
- Mild uterine cramping: normal as the uterus begins growing.
Concerning symptoms in this window: heavy bleeding (more than a pad per hour), severe one-sided pain (possible ectopic pregnancy, which peaks in risk before 8 weeks), or fever. These warrant immediate contact with your provider.
Weeks 9–13: end of the first trimester
Week 9
The fetus is 23–28 mm. The liver is producing blood cells (it will do so until the bone marrow takes over later in pregnancy). Teeth buds are forming. The intestines, which were temporarily herniated into the umbilical cord during rapid development, are moving back into the abdominal cavity.
Week 10
All ten fingers and toes are present and separated. Fingernails begin forming. The fetus can make small spontaneous movements — not yet perceptible to the mother. Genitalia are differentiating, though typically not distinguishable by standard ultrasound this early.
Weeks 11–12
Nuchal translucency (NT) screening — part of the combined first-trimester screen — is performed between 11 weeks 0 days and 13 weeks 6 days (ACOG Practice Bulletin 226, 2021). NT measures the fluid collection at the back of the fetal neck; increased thickness is associated with chromosomal abnormalities including Down syndrome. Combined with blood tests for free beta-hCG and pregnancy-associated plasma protein A (PAPP-A), the detection rate for trisomy 21 is approximately 82–87%.
NIPT (non-invasive prenatal testing) can be performed from 10 weeks onward. It analyzes cell-free fetal DNA in maternal blood and screens for trisomies 21, 18, and 13, sex chromosome conditions, and some microdeletions. Sensitivity for trisomy 21 is greater than 99% with a false-positive rate under 0.1% (ACOG Committee Opinion 762, updated 2022). ACOG recommends offering NIPT to all pregnant patients, not exclusively those over 35.
Week 13
The fetus is approximately 65–78 mm (crown-rump length). It can yawn, suck, and make facial expressions. The placenta has assumed full responsibility for hormone production and oxygen/nutrient delivery. The risk of miscarriage has dropped substantially.
What you might feel, weeks 9–13: Nausea often begins to improve as hCG levels plateau and then decline slightly around week 10–11. Fatigue may persist. Some women notice increased vaginal discharge (leukorrhea), a normal consequence of rising estrogen and increased blood flow to the pelvic area. Round ligament pain — a sharp or stretching sensation at the sides of the lower abdomen — begins as the uterus expands out of the pelvis.
Miscarriage: honest numbers
Miscarriage is common in the first trimester, and most pregnancy apps and books handle the statistics awkwardly — either avoiding them entirely or presenting them in ways that feel threatening without context.
Clinically recognized pregnancies miscarry in approximately 10–20% of cases (ACOG 2018). The risk is not evenly distributed:
- Most losses occur between weeks 5 and 8 — before or around the first prenatal visit.
- After a confirmed fetal heartbeat at 6–8 weeks, ongoing miscarriage risk drops to approximately 5%.
- After a confirmed heartbeat at 8–10 weeks, the risk drops further to approximately 2–3% in low-risk pregnancies.
- After 12 weeks, miscarriage risk is below 1–2% in most populations.
These numbers are not meant to create anxiety, but to provide context. The vast majority of miscarriages in the first trimester result from chromosomal abnormalities — random errors in cell division, not from anything the pregnant person did. They are not caused by exercise, sex, stress, or most foods or activities.
The first prenatal visit
ACOG recommends the initial comprehensive prenatal visit at 8–10 weeks for most patients. If you have had a prior pregnancy loss or are coming from IVF, your provider may schedule an earlier viability scan at 6–7 weeks.
The 8–10 week visit typically includes:
- Medical history and physical exam
- Blood work: blood type and Rh factor, complete blood count (CBC), rubella immunity, HIV, syphilis, hepatitis B surface antigen, varicella immunity, urine culture, and (in many practices) thyroid-stimulating hormone (TSH) and chlamydia/gonorrhea
- Dating ultrasound: CRL measurement to confirm gestational age. In IVF pregnancies, gestational age is already established; ultrasound confirms viability and fetal number rather than revising the date.
- Genetic carrier screening: can be offered at any point preconceptionally or in the first trimester; screens for conditions like cystic fibrosis, spinal muscular atrophy, fragile X, and others depending on the panel
- Discussion of prenatal screening options: NT ultrasound plus blood tests (11–14 weeks), NIPT (from 10 weeks), or integrated/sequential screening
Use the Pregnancy Week Calculator to track exactly where you are each week, and the Due Date Calculator to confirm your estimated due date based on LMP or IVF transfer date.
Symptoms that warrant a call to your provider
The following symptoms are not typical and warrant a prompt call or visit:
- Heavy vaginal bleeding (soaking a pad; more than light spotting)
- Severe abdominal pain or severe one-sided pelvic pain
- Fever over 100.4°F (38°C)
- Signs of dehydration from severe vomiting (inability to keep any fluids down for 24 hours)
- Passage of tissue
- Sudden disappearance of pregnancy symptoms combined with any of the above
Light spotting, especially around weeks 4–6 (implantation) and after pelvic exam or sex, is common and usually benign. But when in doubt, call your provider rather than waiting to see if it resolves.
The bottom line
The first trimester is a thirteen-week stretch of extraordinary development compressed into a period when most people still look and feel, to the outside world, exactly as they did before pregnancy. Symptoms often peak at weeks 7–9 and ease by weeks 10–12. Miscarriage risk is real but declines sharply after a confirmed heartbeat. The first prenatal visit at 8–10 weeks is the right time to establish care, confirm dates, order labs, and discuss screening options.
Track your current week with the Pregnancy Week Calculator, and bring any questions about your specific symptoms or risk factors to your provider rather than relying solely on population statistics. Your individual circumstances are what your prenatal care is designed to address.