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Your Week-by-Week Pregnancy Guide: What to Expect From Conception to Birth

Week-by-week pregnancy development illustration showing fetal growth milestones from conception to birth
Medical Disclaimer: This article is for informational purposes only and does not constitute medical advice. Pregnancy involves significant individual variation — consult your OB-GYN, midwife, or healthcare provider for guidance specific to your situation. Contact your provider immediately if you experience warning symptoms listed in this article.

A full-term pregnancy spans 40 weeks from the last menstrual period — roughly 38 weeks of actual fetal development. Here’s what the research shows is happening at each stage, what symptoms are expected versus concerning, and which calculators on this site are most useful when.

How Pregnancy Weeks Are Counted

Gestational age is measured from the first day of the last menstrual period (LMP), not from conception. This creates a counterintuitive numbering: at the 2-week mark on the gestational calendar, most people haven’t conceived yet. Conception typically occurs around day 14 in a standard 28-day cycle.

Use the due date calculator to establish your gestational age from LMP, or from conception date if you know it. The tool calculates using Naegele’s rule (LMP + 280 days), the clinical standard, with adjustments for cycle length variations.

First Trimester (Weeks 1–12)

Weeks 1–4: Conception and Implantation

Ovulation typically occurs at week 2. Fertilization, if it occurs, happens in the fallopian tube within 24 hours of ovulation. The fertilized egg takes 6–12 days to travel to the uterus and implant in the endometrial lining — typically around days 20–24 of the cycle, or gestational weeks 3–4.

Implantation bleeding — light spotting lasting 1–2 days — occurs in approximately 25% of pregnancies and is often mistaken for a light period. A home pregnancy test can detect hCG (human chorionic gonadotropin) reliably from the date of the expected period onward, or approximately gestational week 4.

Weeks 5–6: Early Embryonic Development

The embryo is approximately 1–5 mm. The neural tube — which will become the brain and spinal cord — forms during weeks 3–4 post-conception (gestational weeks 5–6). This is why folate supplementation must begin before conception: neural tube closure is complete before most people know they are pregnant. The cardiac structures begin forming; a flicker of cardiac activity is detectable on transvaginal ultrasound by week 6.

Common symptoms at this stage: breast tenderness, fatigue (significant — progesterone levels are rising sharply), nausea beginning around week 6 for most people. The nausea signal is driven by hCG, which peaks at weeks 10–12; this is why nausea generally peaks and then resolves in the second trimester.

Weeks 7–10: Organogenesis

This is the period of greatest teratogenic sensitivity — the window when medications, alcohol, and other environmental exposures carry the highest risk of structural birth defects, because all major organ systems are forming simultaneously. The embryo transitions to “fetus” status at week 10 when the critical period of organogenesis is largely complete.

By week 10, the fetus is approximately 3 cm crown-to-rump. Fingers, toes, facial features, and external genitalia are forming. The heart has four chambers and beats at 150–170 bpm — nearly twice adult resting rate, which is developmentally normal and will gradually slow through gestation.

Weeks 11–12: End of First Trimester

The risk of miscarriage drops sharply after week 10. Approximately 80% of miscarriages occur in the first trimester; the rate after week 10 is approximately 1–5% for low-risk pregnancies. The nuchal translucency scan — the first-trimester anatomy screen — is typically performed at weeks 11–13.

Cell-free fetal DNA (cfDNA) screening, which analyzes fetal DNA fragments in maternal blood, is typically offered from week 10 onward. Sensitivity for trisomy 21 (Down syndrome) exceeds 99% with false-positive rates below 0.1% — superior to the older quad screen. Positive cfDNA screening results are followed by diagnostic testing (amniocentesis or CVS) for definitive diagnosis.

Second Trimester (Weeks 13–26)

Weeks 13–16: Relief and Quickening

For most people, the second trimester brings significant symptom relief — nausea typically resolves by week 14, energy returns, and the miscarriage risk is substantially lower. The uterus expands above the pubic symphysis and becomes palpable abdominally.

Fetal movement (quickening) is first felt between weeks 16–25. Multiparous individuals (those who have been pregnant before) often notice movement earlier — as early as week 14–16 — because they recognize the sensation. First-time parents typically feel it between weeks 18–22.

Weeks 17–20: Anatomy Scan

The anatomy ultrasound at weeks 18–22 is the most comprehensive fetal evaluation in the standard prenatal schedule. It assesses:

  • Brain, spine, and facial structures
  • Heart structure (four-chamber view and outflow tracts)
  • Abdominal organs and diaphragm
  • Limb length and bone structure
  • Placental location (identifying placenta previa)
  • Amniotic fluid volume
  • Cervical length (a predictor of preterm birth risk)

Sex assignment, if desired, is typically visible at this scan. Use the due date calculator to confirm your gestational age for scheduling.

Weeks 21–26: Fetal Lung Development Begins

Lung maturation is a late-pregnancy event — surfactant production begins in earnest around week 22–24 but doesn’t reach functional levels for independent breathing until approximately week 34–36. This is why premature birth at 28–32 weeks typically requires respiratory support even with otherwise intact development.

At week 22, survival with intensive NICU care is possible but outcomes vary significantly by hospital. At week 24, survival rates reach 50–70% with appropriate care. Every week of gestation after 22 materially improves lung function and survival probability.

The glucose challenge test (GCT) for gestational diabetes is scheduled at weeks 24–28. Approximately 6–9% of pregnancies develop gestational diabetes; rates are higher in those with BMI over 30, family history of diabetes, or prior GDM diagnosis.

Third Trimester (Weeks 27–40)

Weeks 27–30: Rapid Growth Phase

The fetus gains approximately half its birth weight between weeks 27 and 40. At week 27, average fetal weight is approximately 900g; at term (40 weeks), average birth weight is 3,400g (7.5 lbs). This growth requires significant caloric and nutritional support — iron requirements double in the third trimester as fetal iron stores build.

Fetal movement should be well-established by week 28. Standard clinical guidance from ACOG recommends tracking fetal movement from week 28 onward. The kick counter tool on this site tracks movements and timestamps them. Most guidelines consider 10 movements within a 2-hour window reassuring.

Weeks 31–36: Positioning and Final Organ Maturation

Most fetuses are vertex (head-down) by week 32–36. Breech presentation after week 36 is present in approximately 3–4% of term pregnancies. An external cephalic version (ECV) — manual attempt to turn the baby — can be attempted at weeks 36–37 with approximately 60% success rate.

Lung surfactant production reaches functional levels around week 34–36, which is why “late preterm” infants (34–36 weeks) typically require only brief respiratory support. Brain myelination — the insulation of nerve fibers that enables rapid neural transmission — continues through term and into the first year after birth.

Track your weight gain trajectory against IOM guidelines using the pregnancy weight gain calculator. The third trimester accounts for approximately half of total gestational weight gain.

Weeks 37–40: Full Term

ACOG revised the definition of “full term” in 2013 to distinguish:

  • Early term: 37 0/7 through 38 6/7 weeks
  • Full term: 39 0/7 through 40 6/7 weeks
  • Late term: 41 0/7 through 41 6/7 weeks
  • Post-term: 42 weeks and beyond

Outcomes are measurably better at 39–40 weeks than 37–38 weeks; elective delivery before 39 weeks without medical indication is not recommended. Most guidelines offer induction of labor at 41 weeks due to increasing placental insufficiency risk post-term.

Warning Signs to Know at Every Stage

Contact your provider immediately for any of the following, regardless of gestational age:

  • Vaginal bleeding (any amount in second or third trimester)
  • Severe abdominal pain or cramping
  • Symptoms of preeclampsia: severe headache, visual changes, upper right abdominal pain, facial or hand swelling
  • Decreased fetal movement after week 28 (fewer than 10 movements in 2 hours)
  • Signs of preterm labor before 37 weeks: regular contractions more than 4 per hour, pelvic pressure, watery discharge
  • Symptoms of ruptured membranes: a gush or steady trickle of fluid

The contraction timer tracks contraction frequency, duration, and interval — the three measurements your provider will ask about if you call reporting contractions.

Frequently Asked Questions

Gestational age is counted from the first day of your last menstrual period (LMP), not from conception. This means most people are already 2 weeks 'pregnant' by the standard count before conception even occurs. Conception typically happens around day 14 of a 28-day cycle. This is why a 40-week pregnancy includes approximately 38 weeks of actual fetal development — the first 2 weeks are the late follicular phase before ovulation.
Nausea typically begins around week 6, peaks at weeks 8–10, and resolves for most people by week 12–14. The term 'morning sickness' is misleading — nausea can occur at any time of day. About 0.3–3% of pregnant people develop hyperemesis gravidarum (HG), a severe form involving persistent vomiting that requires medical treatment. Nausea that continues beyond week 16 or is severe enough to prevent adequate hydration warrants medical evaluation regardless of timing.
The threshold of viability — the gestational age at which survival with intensive medical support becomes possible — is generally 22–24 weeks, with outcomes highly variable at 22–23 weeks. Survival rates at 24 weeks reach approximately 50–70% with NICU support; at 28 weeks, survival exceeds 90%. Each additional week of gestation meaningfully improves outcomes through lung maturation and brain development. Infants born before 37 weeks are classified as preterm; before 28 weeks as extremely preterm.
IOM (Institute of Medicine) guidelines from 2009 — still the clinical standard — recommend: underweight (BMI <18.5): 28–40 lbs; normal weight (BMI 18.5–24.9): 25–35 lbs; overweight (BMI 25–29.9): 15–25 lbs; obese (BMI ≥30): 11–20 lbs. These ranges are guidelines, not targets — individual variation is significant. The weight gain pregnancy calculator on this site uses IOM formulas to track your trajectory by week.
First-time parents typically feel fetal movement (quickening) between weeks 18–25. Those who have been pregnant before often notice movement earlier, from week 16 onward. By week 28, the standard clinical recommendation is to track kick counts — most guidelines suggest 10 movements within a 2-hour window as reassuring. A decrease in previously established movement patterns at any point is reason to contact your provider, regardless of gestational age.
First trimester (weeks 10–13): cell-free DNA screening (detects chromosomal abnormalities with 99%+ sensitivity for trisomy 21), nuchal translucency ultrasound. Second trimester (weeks 15–20): quad screen blood test, anatomy ultrasound (week 18–22). Third trimester (weeks 24–28): glucose challenge test for gestational diabetes; weeks 35–37: Group B Strep (GBS) culture. Diagnostic tests (amniocentesis, chorionic villus sampling) are invasive with small miscarriage risk and are typically offered when screening shows elevated risk.

Calculate Your Due Date

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