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Pregnancy Trimesters: Week-by-Week Guide

A typical pregnancy lasts ~40 weeks (counted from the first day of the last menstrual period / LMP), divided into three trimesters. Fertilization actually occurs around week 2, so the embryo/fetus is always about 2 weeks younger than the gestational age.

"A trimester is a period of 3 months, one third of the 9-month period of gestation. The most critical stages of development occur during the first trimester (13 weeks), when embryonic and early fetal development is occurring." - The Developing Human: Clinically Oriented Embryology

🟢 FIRST TRIMESTER — Weeks 1 to 12

This is the most critical period. Major organs and body systems form. Most birth defects (teratogenic effects) occur during this window.

Week 1-2 — Fertilization & Implantation

Sperm fertilizes the egg in the fallopian tube, forming a zygote
Rapid cell division (cleavage) forms a morula, then a blastocyst
Blastocyst implants into the uterine endometrium around day 6-10
No visible embryo yet; pregnancy test becomes positive around week 4 (LMP)

Week 3 — Gastrulation

Three germ layers form: ectoderm, mesoderm, endoderm
Neural tube (future brain and spinal cord) begins forming
Heart cells start developing
Embryo is about 1.5-2.5 mm

Week 4 — Organogenesis Begins

Heart begins beating (cardiac contractions start ~day 22-23)
Neural tube closes
Arm and leg buds appear
Embryo looks like a curved "C" shape, size of a poppy seed (~2-4 mm)

Week 5-6 — Facial Features Begin

Brain divides into 5 vesicles
Eyes, ears, and nose start forming as pits/placodes
Limb buds elongate; paddle-shaped hand plates visible
Heart has 4 chambers forming
Size: sesame seed to lentil (~4-6 mm)

Week 7-8 — End of Embryonic Stage

Fingers and toes are forming (webbed at first)
Eyelids begin forming
All major organs are present in rudimentary form
Embryo develops distinctly human features
Embryonic development ends at stage 23 (~day 56/week 8)
Size: raspberry (~15-22 mm)

Week 9-10 — Fetal Period Begins

Now called a fetus (not embryo)
Fingers and toes are separated
External genitalia beginning to differentiate (sex not yet visible on ultrasound)
Fetus can make small movements
Swallowing begins; kidneys produce urine
Size: strawberry (~30-40 mm)

Week 11-12 — End of First Trimester

All organ systems present and functioning at a basic level
Facial features more refined; eyelids fused
Fingernails forming
Risk of miscarriage drops significantly after this point
Size: plum (~50-60 mm); weight ~14-45 g
Nuchal translucency ultrasound done at 11-13 weeks

Maternal changes (Trimester 1): Morning sickness, breast tenderness, fatigue, frequent urination, mood changes, missed period. hCG levels peak around weeks 8-10.

🟡 SECOND TRIMESTER — Weeks 13 to 27

Often called the "honeymoon trimester" - nausea usually eases, energy returns. Fetus grows rapidly and senses develop.

Week 13-16 — Rapid Growth

Skeleton ossifying (hardening from cartilage to bone)
Fine hair (lanugo) begins to appear
Meconium (first stool) begins accumulating
Fetus starts making sucking movements
Sex can often be determined on ultrasound by week 16
Size: lemon to avocado (~75-115 mm)

Week 17-20 — Quickening (First Movements Felt)

Mother begins to feel fetal movements ("quickening") around weeks 18-20
Vernix caseosa (protective white coating) covers the skin
Eyebrows and eyelashes forming
Fetus can hear sounds from outside the womb
Anatomy ultrasound (anomaly scan) done at 18-20 weeks
Size: banana (~16 cm); weight ~300 g

Week 21-24 — Viability Zone

Skin appears reddish, wrinkled, with visible veins through translucent skin
Eyelids begin to open (week 26)
Fetus responds to sounds with movement and increased pulse
Lungs producing surfactant (from ~week 24)
Viability threshold: ~22-24 weeks - fetus may survive with intensive care if born now
By end of week 24: ~30 cm long, ~600 g

Week 25-27 — End of Second Trimester

Brain developing rapidly; sleep-wake cycles appear
Fat deposits beginning to form under the skin
Responds to pain and light
Amniotic fluid begins to diminish slightly
Size: ~34-37 cm; weight ~900 g-1 kg

Maternal changes (Trimester 2): Belly visibly growing, round ligament pain, back ache, leg cramps, possible gestational diabetes screen (~week 24-28), increased appetite.

🔴 THIRD TRIMESTER — Weeks 28 to 40

Finishing touches on all organ systems; major weight gain. The lungs are last to mature.

Week 28-32 — Brain & Lung Development

Brain develops most rapidly in this period
Fetus can see and hear
Most internal systems well-developed; lungs still maturing
Fetus changes position frequently; may turn head-down (cephalic)
Weight: ~1.5-2.5 kg; length ~40-45 cm
If born premature at 28-32 weeks, survival rate is high with NICU care

Week 33-36 — Lung Maturation

Lungs close to fully developed by week 36
Brain and nervous system continue developing
Lanugo (fine hair) begins to disappear
Toenails and fingernails fully formed
Weight: ~2.5-3.5 kg; length ~45-48 cm
Fetus settles into head-down position for birth (engagement)

Week 37-40 — Term

Week 37+ = Early term; Week 39-40 = Full term
Lungs fully mature, surfactant production complete
Vernix mostly absorbed into skin
Skull bones still soft and unfused (fontanelles) to allow passage through birth canal
Fetus mostly full-grown; puts on ~30 g/day in final weeks
Weight: ~3-4 kg; length ~48-52 cm
Labor usually begins between weeks 39-41

Maternal changes (Trimester 3): Braxton-Hicks contractions, heartburn, shortness of breath (baby pressing on diaphragm), frequent urination returns, swollen ankles, nesting instinct.

Summary Table

Trimester	Weeks	Key Theme	Fetal Size (end)
First	1-12	Organogenesis - all organs form	Plum (~6 cm, 45 g)
Second	13-27	Growth, senses develop, viability	Cabbage (~37 cm, 1 kg)
Third	28-40	Maturation, weight gain, lungs ripen	Watermelon (~50 cm, 3-4 kg)

Key Prenatal Visits & Screenings by Trimester

Timing	Screening/Visit
Weeks 6-8	First ultrasound (confirm pregnancy, heartbeat)
Weeks 10-12	NIPT (non-invasive prenatal testing) for chromosomal abnormalities
Weeks 11-13	Nuchal translucency scan
Week 16	Maternal serum alpha-fetoprotein (quad screen)
Weeks 18-20	Anatomy/anomaly ultrasound
Weeks 24-28	Glucose tolerance test (gestational diabetes screen)
Week 28	Rhesus (Rh) factor check; Anti-D injection if Rh-negative
Weeks 36+	Group B Streptococcus (GBS) swab
Weeks 38-40	Biweekly/weekly visits; cervical assessment

Sources: The Developing Human: Clinically Oriented Embryology (Moore et al.), Cleveland Clinic Fetal Development, Mayo Clinic Pregnancy Week-by-Week.

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