The first trimester — weeks 1 through 13 — is the most critical period of your pregnancy. In just 13 weeks, a single fertilised egg transforms into a fully formed foetus with a beating heart, developing brain, fingers, and the foundations of every major organ. Here is everything you need to know, week by week.
Weeks 1–2: Before Conception
Your body is preparing
Obstetricians count pregnancy from the first day of your last menstrual period. Week 1 is your menstrual cycle; the uterus renews its lining. By week 2, ovulation occurs — the egg matures and the body signals "we're ready." The greatest chance of conception is on ovulation day and 1–2 days before.
Start now: If you haven't begun taking folic acid, start immediately. 400 mcg daily is critical for your baby's neural tube development from the very first days after conception.
Weeks 3–4: Conception and Implantation
A new life begins
After the egg and sperm fuse, a zygote forms and travels to the uterus, dividing rapidly into a blastocyst of ~100 cells. By week 4, the embryo burrows into the uterine lining — this is implantation. The trophoblast starts producing hCG (the pregnancy hormone), and the first three germ layers form: ectoderm (skin, nervous system), mesoderm (bones, heart, muscles), and endoderm (internal organs).
hCG only starts rising once implantation happens, which is why a test taken too early reads negative even in a healthy pregnancy. Our early pregnancy symptoms quiz works out when a test will actually be reliable for your cycle, and explains why the symptoms of these two weeks are indistinguishable from an ordinary period approaching.
Week 5: The Heart Starts Beating
Neural tube and first heartbeats
Neurulation begins — your baby's back folds into a groove that closes into the neural tube, the foundation of the brain and spinal cord. The heart is now S-shaped and making its first arrhythmic pushes. This is why folic acid is so critical: if the neural tube doesn't close completely, serious defects can occur.
Symptom alert: hCG levels are rising fast. You may experience morning sickness, food aversions, breast tenderness and fatigue — all normal signs your pregnancy is progressing.
Week 6: Size of a Pea — 150 Beats per Minute
Eyes, ears, and a racing heart
Your baby is now about the size of a pea. The heart beats at ~150 bpm — twice as fast as yours. Dark spots appear on the head: future eyes. Tiny pits mark where the ears will be. The arms and legs look like small buds. Inside, the first neural connections (synapses) are forming.
Week 7: 100,000 New Neurons Every Minute
Brain building at extraordinary speed
The brain is developing faster than any other structure — 100,000 new neurons appear every minute. Tiny nostrils form; arms grow longer. The baby makes its first movements (too light to feel yet). The kidneys are in place and preparing for their first job. A mucus plug forms in the cervix, protecting the pregnancy from infection.
Week 8: The Embryo Graduates to Foetus
Four-chamber heart, separating fingers
This week marks the transition from embryo to foetus. Arms and legs lengthen; the webbing between fingers begins separating. Ears take shape; an upper lip and nose tip appear. The heart now has four chambers, beating rhythmically. The gonads start differentiating into testes or ovaries.
What to do: Book your first prenatal appointment if you haven't already. Discuss any chronic conditions and medications — organ formation is happening right now.
Weeks 9–11: Official Foetus
Taste buds, swallowing, and growing fast
At week 9 the baby is officially a foetus. Taste buds develop — it can already sense what you eat through amniotic fluid. By week 10, the finger webs fully disappear; the tail is gone; all 20 milk teeth are laid down in the gums. At week 11, the foetus actively swallows amniotic fluid to train the lungs and intestines. External genitals begin forming.
Weeks 12–13: The End of the First Trimester
Miscarriage risk drops significantly
By week 12, the foetus is fully formed with all organs in place. The risk of miscarriage drops to under 2%. The placenta takes over full hormone production. At week 13 your baby can suck its thumb. Most countries offer the first-trimester screening scan (nuchal translucency) between weeks 11 and 13 to check for chromosomal conditions — our guide to first trimester screening explains how that chance figure is produced and what the 1 in 150 cut-off means. If you have had an earlier scan before this one, it was looking for something different.
Common First Trimester Symptoms
- Morning sickness and nausea (often strongest at weeks 8–10)
- Extreme fatigue — your body is building a placenta from scratch
- Breast tenderness and enlargement
- Frequent urination as the uterus presses on the bladder
- Food cravings and aversions driven by hCG
- Heightened sense of smell
- Acne and skin breakouts as androgen levels climb
- Spotting (implantation bleeding) in weeks 3–4 — usually normal
- Constipation, often the first hint that digestion has slowed
Always contact your doctor if you experience heavy bleeding, severe one-sided abdominal pain, or fever above 38°C.
Essential Supplements in the First Trimester
The first trimester is when your baby's brain, heart and spine are forming — nutrition matters enormously, and so does knowing which foods to avoid while organ formation is under way.
- Folic Acid 400 mcg/day — prevents neural tube defects. Start before conception and continue through week 12 at minimum.
- Iodine 220 mcg/day — regulates thyroid hormones that drive fetal brain development. Critical all trimesters.
- Vitamin D 600 IU/day — supports immune function; deficiency is very common, especially in northern regions.
Read the full supplements guide: Prenatal Supplements Guide →
First Trimester Checklist
- Start or continue folic acid 400 mcg daily
- Book your first prenatal appointment (ideally by week 8–10)
- Get blood tests: CBC, ferritin, TSH, blood type, Rh factor
- Schedule the 11–13 week nuchal translucency scan
- Avoid alcohol, smoking, raw fish, and unpasteurised dairy
- Eat small, frequent meals to manage nausea
- Drink 1.5–2 litres of water daily
- Avoid hot baths and X-rays
Two habits started now pay off later: gentle exercise, which is safe from the first trimester in an uncomplicated pregnancy, and getting used to side-sleeping before the bump makes it necessary. For the full arc from here to birth, the week-by-week guide covers every week individually, the pregnancy calendar puts your own dates against every appointment in it, and baby size week by week shows how far a first-trimester embryo still has to go. If you are working backwards rather than forwards, the conception date calculator explains why you were counted as two weeks pregnant on the day of conception.
Continue your journey: Second Trimester Guide: Weeks 14–27 →
FAQ — First Trimester
When does the first trimester start and end?
The first trimester runs from week 1 through week 13, counted from the first day of your last menstrual period rather than from conception — so by the time a pregnancy test turns positive around week 4, the embryo itself is closer to two weeks old. It ends once the placenta has fully taken over hormone production and the risk of miscarriage drops sharply, usually around week 12–13.
Why is early miscarriage more common in the first trimester?
Most first-trimester miscarriages happen because of a chromosomal issue in the embryo itself, not something the mother did or did not do. The risk drops substantially — to under 2% — once the pregnancy passes week 12, which is one reason many people wait until after the first-trimester scan to share the news more widely, though that timing is a personal choice, not a medical requirement.
Is it normal to have no symptoms at all in the first trimester?
Yes — symptom intensity varies enormously and has no bearing on how well the pregnancy is progressing. Some people have severe nausea and exhaustion from week 5, others notice almost nothing beyond a missed period until the first scan. Neither pattern is a sign of anything being wrong on its own.
What is the single most important thing to do in the first trimester?
Starting or continuing 400 mcg of folic acid daily, ideally from before conception, since it is critical for neural tube development in exactly this window. Booking the first prenatal appointment by week 8–10 is the second priority — earlier is better than later if there is any uncertainty about dates or risk factors.