The first trimester spans from the first day of your last menstrual period (LMP) through the end of week 13 — a period of extraordinary biological change. Most of the baby's critical organ systems form in these 13 weeks, which is why the first trimester carries the highest risk of miscarriage and why certain medications and exposures are most harmful during this window.
Weeks 1–2: The Waiting Period
Technically, pregnancy weeks 1–2 occur before conception. Gestational age is counted from the first day of the last menstrual period — not from fertilisation — because ovulation and conception dates are rarely known precisely.
During weeks 1–2, the uterine lining is rebuilding following the previous period, and ovulation is approaching.
Weeks 3–4: Fertilisation and Implantation
Week 3: An egg is fertilised, creating a single-celled zygote. The zygote divides rapidly as it travels down the fallopian tube, becoming a blastocyst of about 100 cells by the time it reaches the uterus.
Week 4: The blastocyst implants into the uterine lining. The placenta and amniotic sac begin forming. The hormone hCG (human chorionic gonadotropin) is released — the hormone detected by home pregnancy tests.
Symptoms: A missed period. Some women notice light implantation spotting. Breast tenderness may begin.
Weeks 5–6: The Embryonic Stage Begins
Week 5: The embryo is about 2 mm long — the size of an apple seed. Three layers of cells (ectoderm, mesoderm, endoderm) are forming that will become every organ and tissue in the body.
Week 6: A primitive heartbeat is detectable via transvaginal ultrasound — typically around 90–110 bpm at this stage. The neural tube, which will become the brain and spinal cord, is forming.
Symptoms: Nausea (often called morning sickness) typically begins around week 6, though it can occur at any time of day. Fatigue, breast soreness, and frequent urination are common.
Weeks 7–10: Rapid Organ Development
| Week | Size | Key Development |
|---|---|---|
| 7 | Blueberry (10 mm) | Face forming, arm and leg buds appear |
| 8 | Raspberry (16 mm) | Fingers and toes forming, all major organs present in basic form |
| 9 | Grape (23 mm) | Embryo becomes a fetus; external genitalia beginning to differentiate |
| 10 | Kumquat (30 mm) | Heartbeat audible via Doppler; bones beginning to harden |
Week 10 milestone: The embryonic period ends and the fetal period begins. This is significant — the risk of major structural birth defects from external exposures drops considerably after this point, as organ formation is largely complete.
Week 11–13: The First Trimester's End
Week 11: The fetus can make spontaneous movements, though they are too small to feel. Fingernails are forming.
Week 12: Reflexes develop. The fetus can open and close fingers, curl toes, and make sucking movements.
Week 13: The end of the first trimester. The fetus is approximately 7–8 cm (crown to rump) and weighs about 23 grams. The risk of miscarriage drops significantly after this point.
Key Prenatal Tests in the First Trimester
| Test | Timing | Purpose |
|---|---|---|
| Transvaginal ultrasound | 6–8 weeks | Confirm pregnancy, heartbeat, location |
| Blood type & Rh factor | 8–10 weeks | Identify Rh incompatibility risk |
| NT scan (Nuchal Translucency) | 11–13 weeks | Screens for Down syndrome and other chromosomal conditions |
| Cell-Free DNA (cfDNA) / NIPT | 10+ weeks | Non-invasive chromosomal screening |
| CVS (Chorionic Villus Sampling) | 10–13 weeks | Diagnostic (if indicated) |
Knowing Your Due Date
Understanding your due date and current gestational week helps you track all these milestones against the right timeline. Use our Pregnancy Due Date Calculator to calculate your EDD from your LMP and see which trimester week you are currently in.
This article is for informational purposes. Always consult your obstetrician or midwife for personalised prenatal guidance.