The first trimester, weeks 1 to 13
The weeks when the most happens and the least shows. Here is what is going on, and the few things that really matter.
Updated October 13, 2026 · sources checked October 13, 2026
In short
The first trimester runs from the first day of your last period to 13 weeks and 6 days. It is when your baby's brain, spine, heart and organs begin to form. The essentials: take folic acid, stop alcohol, book your first appointment early, and expect a dating scan between 10 and 14 weeks. Tiredness and nausea are common and usually ease by the second trimester.
How the first trimester is counted
Pregnancy is dated from the first day of your last menstrual period, not from conception. ACOG explains that this adds about two weeks at the start, when you are not yet pregnant. The first trimester ends at 13 weeks and 6 days. If you know the date of your last period, our pregnancy week calculator tells you exactly where you are today.
What is happening for your baby
ACOG describes these milestones:
| Weeks | Development |
|---|---|
| 1 to 8 | Brain and spine begin to form; heart tissue starts to develop; muscles of the eyes, nose and mouth form; the inner ear begins to develop |
| 9 to 12 | Cartilage for limbs, hands and feet forms; eyelids form; kidneys begin making urine; the pancreas starts making insulin; fingernails form |
ACOG also notes that the embryo is called a fetus from 9 weeks after fertilisation.
What you may feel
The NHS lists the early signs most people notice from around 4 to 6 weeks: a missed period, tiredness, sore or larger breasts, needing to pee more often, nausea at any time of day, a metallic taste, a sharper sense of smell, and mood changes. Our guide to early signs of pregnancy goes through each one.
Morning sickness can strike morning, noon or night. The NHS says it usually clears up by weeks 16 to 20 and does not put your baby at increased risk. See what helps with morning sickness and the signs that it needs medical attention.
The four things that matter most now
- Folic acid. The NHS advises 400 micrograms a day until 12 weeks. A large trial, the MRC Vitamin Study published in 1991, found a 72% protective effect of folic acid against neural tube defects such as spina bifida. Some people need a higher dose: ask your midwife, GP or OB-GYN.
- No alcohol. There is no known safe amount at any stage. More in our guide to caffeine and alcohol in pregnancy.
- Food safety. Listeria and toxoplasmosis matter from the very start. Keep our list of foods to avoid to hand.
- Book your care. In England you can refer yourself to a midwife; in the US, call an OB-GYN or midwife practice. Tell them about any long-term condition or regular medicine straight away.
Tests and appointments in the first trimester
| Test | What it checks | Source |
|---|---|---|
| Blood type and Rh factor | Whether you are Rh negative, which changes your care | ACOG |
| Urine culture | Bacteria that can cause a urinary infection without symptoms | ACOG |
| Infection screening | Including syphilis, and chlamydia and gonorrhoea for some | ACOG |
| Dating scan (England: about 10 to 14 weeks) | How many weeks pregnant you are and your due date | NHS |
| Combined screening test (optional, 10 to 14 weeks) | Chance of Down's syndrome, using a blood test and the nuchal translucency measurement | NHS |
The NHS says the dating scan usually takes about 20 minutes and you may be asked to come with a full bladder. The full schedule for both countries is in our guide to prenatal appointments.
Printable checklist: questions for your first appointment
Is this normal? When to call
Tiredness, nausea, sore breasts, needing to pee more often and big emotions are all normal now. Some spotting can happen too, but always mention it. Call straight away for:
- Vaginal bleeding, tummy pain or pelvic pain.
- Vomiting so severe you cannot keep fluids down for 24 hours, or very dark urine.
- A high temperature.
In the UK, the NHS says to contact your GP surgery or early pregnancy unit in the first 12 weeks; call NHS 111 if you are unsure and 999 in an emergency. In the US, call your OB-GYN or midwife, or 911 in an emergency.
For your partner
The first weeks are often invisible from the outside and exhausting on the inside. Take over cooking if smells are hard, come to the first appointment and the dating scan if you can, and keep the warning signs above in mind. Many people keep the news private until after the scan: follow her lead.
Next: the second trimester
From 14 weeks, energy often returns and your bump starts to show. Here is what the second trimester brings.
Questions parents ask
When does the first trimester end?
ACOG defines the first trimester as running from the first day of your last period to 13 weeks and 6 days. The second trimester starts at 14 weeks.
When should I start taking folic acid?
Ideally before you conceive. The NHS advises taking it until 12 weeks; ACOG advises starting at least a month before pregnancy if possible. Some people need a higher dose, so ask your midwife, GP or OB-GYN.
When is the first ultrasound?
In England the dating scan is offered at around 10 to 14 weeks. In the US the timing depends on your provider; an early ultrasound may be used to estimate your due date if the date of your last period is uncertain.
Is it normal to have no symptoms at all?
Yes. Every pregnancy is different and not everyone has nausea or sore breasts. If you are worried, your midwife or doctor can talk it through with you.
Sources
- ACOG — How Your Fetus Grows During Pregnancy checked October 13, 2026
- ACOG — Routine Tests During Pregnancy checked October 13, 2026
- NHS — Pregnancy vitamins and supplements checked October 13, 2026
- NHS — 12-week scan checked October 13, 2026
- NHS — Vomiting and morning sickness checked October 13, 2026
- NHS — Pregnancy symptoms you need to get help for checked October 13, 2026
- CDC — About Fetal Alcohol Spectrum Disorders checked October 13, 2026
- MRC Vitamin Study Research Group (1991), The Lancet — Prevention of neural tube defects (PMID 1677062) checked October 13, 2026
Mamavela explains; it does not replace your midwife, doctor or pediatrician, who know your situation. If something worries you, call them. In an emergency, call 911 (US), 999 (UK) or 112.