Trimester by Trimester
First Trimester Week by Week: Fetal Development, Weeks 1–13
A midwife-reviewed guide to what your baby is building — and what your body is managing — from a poppy seed to a lime.
Clinically reviewed · June 2026
The first trimester spans weeks 1 through 13 and is the most consequential developmental window of a human life. Every major organ, limb, and body system forms from scratch during these weeks. By week 13, a fully formed — if still very small — fetus exists, and miscarriage risk drops substantially as the most complex developmental work is complete.
The first trimester is counted from the first day of your last menstrual period, which means weeks 1 and 2 technically precede fertilization. This is an obstetric convention designed around the reality that the moment of conception is almost never known precisely, but the date of a period usually is. It can feel strange to be "four weeks pregnant" when the embryo is only about two weeks old — but the math makes sense once you understand the counting convention.
What follows is a week-by-week guide to what is being built, grounded in guidance from ACOG, Mayo Clinic, and Cleveland Clinic — the sources your OB or midwife is working from too.
What Is Happening Week by Week? A Complete First-Trimester Milestone Guide
Weeks 1–2: Before the embryo exists. The body prepares for ovulation. No embryo exists during week 1 (which is your period). Fertilization happens near the end of week 2, when the resulting zygote begins dividing rapidly. There are no pregnancy symptoms because hCG — the hormone that drives them — has not yet risen.
Weeks 3–4: Implantation and the first heartbeat-forerunner. The blastocyst travels down the fallopian tube and implants into the uterine lining, typically 6 to 12 days after fertilization. By week 4, the amniotic sac is forming and the placenta begins developing. The embryo is approximately 2 mm — about the size of a poppy seed. Critically, the neural tube initiates formation during weeks 3–4. The neural tube will become the brain and spinal cord, and it closes by approximately day 28 of embryonic life (week 6 of pregnancy) — before most women know they are pregnant. This is the biological reason folic acid or methylfolate supplementation must begin before conception, not after a positive test.
Week 5: The heart begins pulsing. The primitive heart tube starts pulsing approximately 110 times per minute by the end of week 5. The embryo is roughly sesame-seed sized and still more cellular architecture than recognizable form. Neural tube formation is accelerating.
Week 6: Limb buds and a visible heartbeat. Arm and leg buds emerge. Blood circulation begins. Early ear, eye, and mouth structures form. By week 6, the heartbeat is detectable by transvaginal ultrasound. The embryo is roughly the size of a lentil or small blueberry — about 4 to 5 mm.
Week 7: The brain is building at 250,000 neurons per minute. New neurons form at a rate of approximately 250,000 per minute, according to Cleveland Clinic. The head is disproportionately large relative to the body because the brain is the developmental priority. Cartilage begins hardening, and the embryo still has a small receding tail — a vestige of earlier vertebrate development. The embryo is roughly 1 cm long.
Week 8: All major organ systems are present — and the embryo becomes a fetus. This is the transition week. All major organ systems — heart, brain, liver, kidneys, digestive tract, lungs — are present in at least rudimentary form. Fingers and toes are still webbed. The umbilical cord is fully functional. After week 8, the medical term shifts from embryo to fetus, reflecting that the most foundational phase of organ creation is complete. The fetus is approximately 1.6 cm.
Week 9: Teeth buds and a Doppler-audible heartbeat. Teeth and taste buds begin forming. Muscles develop and begin to move. The heartbeat may become audible via an external Doppler device for the first time. The fetus is approximately grape-sized, roughly 2.5 cm.
Week 10: Fingers fully separated, nails beginning to grow. The webbing between fingers and toes has resolved — digits are fully individuated. Fingernails and toenails begin growing. External ears are taking shape. The fetus is roughly the size of a kumquat, about 3 cm in length.
Week 11: Joints, fists, and bone hardening underway. The fetus opens and closes both fists and mouth. Knees, elbows, and ankles are functional joints. Bone calcification is underway — cartilage is beginning to harden into bone. The skin remains translucent, allowing underlying structures to be visible on ultrasound.
Week 12: Every system present; the fetus actively swallows. By week 12, every organ, limb, bone, and muscle system is present in rudimentary form. The circulatory, digestive, and urinary systems are all operational. The liver is producing bile. The fetus is actively swallowing and excreting amniotic fluid — early practice for the systems it will need after birth. It measures approximately 5.4 cm and weighs about 14 grams — close to the size of a lime.
Week 13: End of the first trimester. Vocal cords form. Bones continue hardening. The intestines begin producing meconium, the initial stool. The head becomes more proportional to the body. According to ACOG, the first trimester officially runs through 13 weeks and 6 days. After week 13, the risk of miscarriage drops substantially, because the most complex developmental work is largely complete.
Miscarriage risk falls from roughly 9.4% at week 6 to approximately 0.5% by week 9. After week 12, the risk is estimated at 1–5% through week 20. The March of Dimes reports that about 80% of all miscarriages occur before week 12 — which is exactly why week 13 feels like a meaningful threshold for many families.
Why Does the First Trimester Matter So Much for Folate Timing?
Folate timing is one of the most important — and most misunderstood — facts in prenatal care. The neural tube, which becomes the brain and spinal cord, closes by approximately day 28 of embryonic life, which is week 6 of pregnancy. Most women discover they are pregnant between weeks 4 and 6. By the time the positive test appears, the neural-tube closure window is already closing.
This is why ACOG recommends that women who may become pregnant take 400–800 mcg of folate daily, beginning at least one month before conception. Folate's protective effect against neural tube defects (NTDs) like spina bifida and anencephaly operates during a narrow window that predates most pregnancy awareness.
A 2024 evidence-based narrative review in Nutrients (Samaniego-Vaesken et al., PMID 39339754) found that both synthetic folic acid and the active methylfolate form (5-MTHF) achieved equivalent reductions in neural tube defect risk. Women with MTHFR gene variants — which slow folic acid conversion — may benefit from choosing a prenatal vitamin that contains 5-MTHF directly, though mainstream ACOG guidance continues to recommend standard folic acid as the primary recommendation for all women.
The practical takeaway: start a prenatal vitamin before you start trying. Waiting until a positive test means the most critical protective window has already passed.
What Symptoms Are Normal in the First Trimester, and When Should You Call Your Provider?
The first trimester often brings a cascade of unfamiliar sensations, most of them driven by rising hCG and progesterone. Understanding which symptoms are expected — and which warrant a call — helps calibrate appropriate concern.
Normal and expected. Nausea affects 70–80% of pregnant women, typically beginning between weeks 4 and 9 and resolving by weeks 12–14 for most. Profound fatigue is driven by progesterone and is normal from as early as weeks 4–6. Breast soreness and darkening areolas are among the most consistent early signs, often appearing between weeks 4 and 6 according to Johns Hopkins Medicine. Frequent urination begins in the first trimester as kidneys filter blood at an accelerated rate and the expanding uterus presses on the bladder. Mild constipation is common throughout the first trimester due to progesterone's relaxation of smooth muscle in the intestinal wall.
Symptoms that warrant a provider call. Vaginal bleeding or spotting of any amount should always be reported, even though many cases of first-trimester spotting resolve without complications. Severe, one-sided abdominal pain is a red-flag symptom that can indicate an ectopic pregnancy rather than a normal intrauterine pregnancy — this is an emergency. Pain radiating to the shoulder tip, fainting, or an inability to retain fluids also require immediate evaluation. A sudden sharp decrease in pregnancy symptoms — particularly nausea and breast tenderness — may occasionally signal a problem and is worth discussing with your provider, though fluctuating symptom intensity is common in normal pregnancies.
This article provides general health information, not medical advice. Talk with your OB-GYN, midwife, or other qualified provider about your individual pregnancy — and always contact your provider promptly with any symptoms that concern you.
Quick-Reference: First Trimester Week-by-Week Development Table
| Week | Approximate Size | Key Development Milestones | Miscarriage Risk (approx.) |
|---|---|---|---|
| 1–2 | No embryo yet | Menstrual period; ovulation at end of week 2; fertilization occurs | N/A (pre-implantation) |
| 3–4 | ~2 mm (poppy seed) | Implantation; amniotic sac forms; neural tube initiation; placenta begins | Elevated (pre-detection) |
| 5 | ~3 mm (sesame seed) | Primitive heart tube pulsing ~110 bpm; neural tube accelerating | High |
| 6 | ~4–5 mm (lentil) | Arm and leg buds; heartbeat detectable via transvaginal ultrasound; ear/eye buds | ~9.4% |
| 7 | ~1 cm (blueberry) | 250,000 neurons/minute; cartilage forming; tail receding; external genitals beginning | ~4.2% |
| 8 | ~1.6 cm | All major organ systems present; fingers/toes webbed; umbilical cord functional; embryo → fetus | ~1.5% |
| 9 | ~2.5 cm (grape) | Teeth and taste buds form; muscles developing; heartbeat audible via Doppler | ~0.5% |
| 10 | ~3 cm (kumquat) | Fingers fully separated (webbing resolved); nails begin growing; ears forming | <1% |
| 11 | ~4 cm | Fists open/close; joints functional (knees, elbows, ankles); bone calcification begins | <1% |
| 12 | ~5.4 cm / 14 g (lime) | All systems present; liver producing bile; fetus swallowing amniotic fluid; circulatory/digestive/urinary operational | <1% |
| 13 | ~7 cm | Vocal cords form; meconium production begins; bones hardening; head proportioning | <1% |
Size comparisons sourced from Femia Health and Cleveland Clinic. Miscarriage risk figures from Medical News Today citing published cohort data; individual risk varies by maternal age and clinical factors. Crown-rump length (CRL) via ultrasound is the clinical gold standard for gestational age measurement.
Frequently asked
What is happening in the first two weeks of pregnancy?
Weeks 1 and 2 of pregnancy, as counted by obstetric convention, do not actually involve an embryo yet. Obstetricians count from the first day of your last menstrual period (LMP), which means week 1 is your period itself and week 2 is the run-up to ovulation. Fertilization typically happens at the very end of week 2. This dating convention exists because LMP is a reliable, observable date — unlike the moment of conception. According to ACOG's fetal development guidance, the fertilized egg (zygote) begins rapid cell division within 24 hours. No pregnancy symptoms are present at this stage because hCG has not yet risen. This is why prenatal vitamins with folate matter before you see a positive test.
When does the neural tube close, and why does that make folate timing so critical?
The neural tube — the structure that becomes the brain and spinal cord — begins forming in weeks 3 and 4 and closes by approximately day 28 of embryonic life, which is around week 6 of pregnancy. Most women do not yet know they are pregnant at this point. This is precisely why ACOG and the CDC recommend beginning folate supplementation before conception, not after a positive test. By the time the pink line appears, the critical neural-tube closure window is already closing. Mayo Clinic's first-trimester guide confirms that the protective window for folate is largely complete by week 6. The standard recommendation is 400–800 mcg of folate daily, starting at least one month before trying to conceive.
What size is the baby week by week in the first trimester?
Fruit comparisons make these tiny measurements feel real. At week 4, the embryo is roughly 2 mm — about the size of a poppy seed. By week 6 it has grown to a lentil or small blueberry (~4–5 mm). Week 7 brings it to a centimeter — about a blueberry. At week 9 it is roughly grape-sized (~2.5 cm). Week 10 is kumquat-sized (~3 cm). By week 12, the fetus measures approximately 5.4 cm and weighs about 14 grams — the size of a lime. These comparisons come from size-by-week data compiled by Femia Health and Cleveland Clinic, cross-referenced against ACOG crown-rump-length (CRL) data. CRL, measured by ultrasound, is the gold-standard measurement method in the first trimester.
When does the embryo become a fetus, and why does the terminology change?
The transition from embryo to fetus happens at week 8 of pregnancy. The distinction is not arbitrary — it reflects a genuine biological turning point. Before week 8, major organ systems are being laid down from scratch (a process called organogenesis). After week 8, those systems are present in at least rudimentary form, and development shifts to growth and refinement rather than origination. Cleveland Clinic's fetal development resource notes that all major organ systems are present by week 8, fingers and toes are still webbed, and the umbilical cord is fully functional. The fetal period from weeks 8 through 40 is characterized by growth and maturation, while the embryonic period (weeks 3–8) is when the foundational blueprint is drawn.
Why does miscarriage risk drop after week 12?
Miscarriage risk is highest in the earliest weeks of the first trimester because the processes underway — organogenesis, neural-tube closure, placental development — are enormously complex, and any disruption can be fatal to the developing embryo. March of Dimes reports that approximately 80% of all miscarriages occur before week 12. Risk is estimated at roughly 9.4% at week 6, falling to about 1.5% at week 8 and 0.5% by week 9. After week 12, the risk drops to 1–5% through week 20. This steep decline reflects the completion of critical organ formation and the establishment of a functioning placenta. Mayo Clinic notes that by week 13, every organ, limb, and muscle system is present, though many require further maturation. Talk with your provider about any spotting or cramping — even in a statistically lower-risk window.
What is happening with the baby's brain in the first trimester?
The brain develops at a pace that is almost impossible to comprehend. By week 7, new neurons are forming at an estimated 250,000 per minute, according to Cleveland Clinic's fetal development data. The head at this stage is disproportionately large relative to the body — because the brain is the priority. The neural tube that closes around week 6 becomes the brain and spinal cord. By week 12, the brain has differentiated into distinct regions, though all higher-function development continues well into the third trimester and beyond. This rapid early neurogenesis is one reason why DHA status in the first trimester matters — research published in Nutrients (the ECLIPSES Study, 2024) found that maternal omega-3 and DHA concentrations in early pregnancy were significantly associated with infant neurodevelopmental outcomes, not only the better-known third-trimester accumulation window. See Cleveland Clinic's full fetal development timeline.
Should I worry about environmental exposures in the first trimester?
The first trimester's extraordinary pace of organogenesis does make it the window of greatest vulnerability to environmental toxins — and this is worth understanding clearly, without alarm. A 2026 review in Current Research in Toxicology found that endocrine-disrupting chemicals including bisphenols (BPA) and phthalates are linked to adverse maternal and fetal outcomes, with the placenta being a specific target of phthalate exposure. Practical, evidence-supported steps include: choosing glass or stainless-steel food storage over plastics labeled 3 or 7, avoiding heating food in plastic containers, selecting fragrance-free personal care products, and filtering drinking water. Choosing organic produce for items on the Environmental Working Group's Dirty Dozen list is a reasonable precaution. This is general information, not medical advice — talk with your provider about individual exposures and do not stop any prescribed medication without guidance.