First Trimester of Pregnancy: What to Expect, Week by Week

Quick answer

The first trimester runs from week 1 to the end of week 13, counted from the first day of your last period. It is when your baby's major organs form, and when you are likely to feel most tired and nauseous. Two things matter most: start folic acid immediately, and register for antenatal care before 12 weeks so your dating scan and baseline tests happen on time.

Key takeaways

• Start folic acid today. It works in the first few weeks, often before many women realise they are pregnant, and it is the single most valuable thing you can do.

• Book antenatal care before 12 weeks. The dating scan, baseline blood tests and the NT scan window are all time-sensitive.

• You do not need extra calories yet. Additional energy needs begin in the second trimester, not now.

• Most of your baby's major organs form in these 13 weeks, which is why medication review and stopping alcohol and tobacco matter more now than at any other stage.

• Know four warning signs: bleeding, severe one-sided abdominal pain, fainting, and being unable to keep fluids down.

What is the first trimester?

The first trimester covers weeks 1 to 13 of pregnancy. Pregnancy is dated from the first day of your last menstrual period, not from conception, which is why you are counted as two weeks pregnant before conception actually happens. It is a convention that confuses almost everyone at first, and your dating scan will confirm the timeline.

This is the shortest trimester but the most consequential developmentally. By the end of it, all of your baby's major organs have formed and the pregnancy moves from building structures to growing them.

It is also, for most women, the hardest trimester to get through. Nausea and exhaustion peak here, there is usually nothing visible to show for it, and many women have not yet told anyone.

Week by week: what is happening

Sizes are approximate. Your baby's measurements are confirmed on scan, and small variations are normal.

Common first trimester symptoms

Nausea affects the majority of pregnancies, though published estimates vary. Having no nausea at all is equally normal and says nothing about the health of your pregnancy.

What changes in your body

• Blood volume begins rising and continues increasing substantially through pregnancy. This contributes to tiredness, breathlessness and dizziness on standing

• Your heart works harder, with heart rate rising gradually

• Progesterone relaxes smooth muscle throughout the body, which explains constipation, heartburn, bloating and the need to urinate more

• Your uterus roughly doubles in size but stays inside the pelvis until around week 12

• Breasts enlarge and veins become more visible

• Your kidneys filter more, increasing urination from early on

You will not usually be showing. Most first-time mothers do not have a visible bump until 12 to 16 weeks. Anything visible before then is almost always bloating, which our guide to what is normal at 10 weeks explains in detail.

What to do in the first trimester

1. Start folic acid

400 micrograms daily, from before conception ideally, and through the first 12 weeks. It reduces the risk of neural tube defects, and the neural tube forms around weeks 5 to 6.

A higher dose applies to some women, including those with diabetes, epilepsy on certain medications, a previous baby with a neural tube defect, a high BMI, or certain absorption conditions. Ask your doctor which applies to you.

If you have only just found out and have not been taking it, start now and do not spend time worrying about the weeks before.

In India, iron and folic acid tablets are provided free at government health facilities.

2. Register for antenatal care before 12 weeks

Early booking matters because several things are time-sensitive.

In India, antenatal care is free at government health facilities, and free check-ups are offered under Pradhan Mantri Surakshit Matritva Abhiyan on the ninth of each month. Your ASHA or ANM worker can help with registration.

One legal point. Determining or disclosing the sex of the baby is illegal in India under the PCPNDT Act. No scan centre will answer that question.

3. Review every medicine and supplement

Bring the actual packets to your first appointment. Include prescription medicines, over-the-counter painkillers, Ayurvedic and homeopathic preparations, protein powders, gym supplements and vitamins.

Paracetamol is the usual first-line painkiller in pregnancy. Avoid NSAIDs such as ibuprofen and diclofenac from 20 weeks, including pain balms and sprays.

Tell every doctor, dentist and pharmacist that you are pregnant. Dental treatment, incidentally, is safe in pregnancy and should not be postponed.

4. Stop alcohol and all tobacco

Including gutkha, khaini and second-hand smoke exposure. If you have a chronic condition such as diabetes, thyroid disease or epilepsy, this is the moment to get it optimised.

5. Eat normally, not for two

The first trimester requires no extra calories. This surprises most people, and knowing it prevents weight gain that causes problems later. Extra energy needs begin in the second trimester.

Focus on quality: protein at every meal, dal, curd, eggs or paneer, vegetables, fruit, whole grains and nuts. If nausea means you eat badly for a few weeks, that is acceptable. Keep taking folic acid and eat what you can keep down.

What to avoid eating

A short list. Most of your normal diet is fine.

• Unpasteurised milk and products made from it, including loose paneer and khoya of uncertain origin

• Raw or undercooked eggs and meat

• Raw or undercooked fish and shellfish, and high-mercury fish such as shark, swordfish and king mackerel

• Cut fruit and salads left standing, particularly from roadside vendors

• Raw sprouts and unwashed produce

• Animal liver, which is very high in vitamin A

• Alcohol, entirely

• Caffeine above about 200 mg a day, roughly two cups of coffee

• Concentrated herbal and unregulated Ayurvedic preparations

Most traditionally forbidden foods are safe. Pineapple, ripe papaya, drumstick and sesame do not cause miscarriage, which our guide to foods and miscarriage covers honestly.

Exercise, work, travel and sex

About miscarriage

Most women worry about this in the first trimester, and it is worth addressing directly rather than avoiding.

Miscarriage is common in early pregnancy, and the great majority of losses are caused by chromosomal abnormalities in the embryo. These occur randomly at conception and cannot be prevented by diet, rest, avoiding stairs, or anything else you do or do not do.

Things that do not cause miscarriage: lifting a normal bag, climbing stairs, exercising in an uncomplicated pregnancy, having sex, a stressful day, an argument, or eating pineapple or papaya.

Things worth doing: folic acid, controlling diabetes and thyroid disease, stopping alcohol and tobacco, keeping caffeine under 200 mg, and reviewing your medication.

If you have had a loss before, tell your doctor at the first visit so your care can be planned accordingly.

Warning signs: when to call a doctor

Seek care immediately if you have:

• Vaginal bleeding, whether spotting or heavier

• Severe abdominal pain, especially on one side

• Pain at the tip of your shoulder, which with abdominal pain can indicate an ectopic pregnancy

• Fainting, dizziness or collapse

• Vomiting so persistent you cannot keep fluids down, or passing very little or dark urine

• Fever

• Burning urination with fever or pain in your side

• Severe headache with visual changes

Book an appointment for: weight loss continuing after nausea settles, persistent low mood or anxiety, any chronic condition not yet reviewed, or anything that worries you.

Our full guide to pregnancy warning signs and when to call a doctor covers these by urgency.

Frequently asked questions

How many weeks is the first trimester?
Weeks 1 to 13, counted from the first day of your last menstrual period. Your dating scan will confirm the timeline.

What is the most important thing to do in the first trimester?
Start folic acid and register for antenatal care before 12 weeks. Everything else is management around those two.

Do I need extra calories in the first trimester?
No. Additional energy needs begin in the second trimester. Focus on the quality of what you eat rather than the quantity.

When is the first scan?
Usually a dating scan between 6 and 12 weeks. The NT scan follows between 11 weeks 0 days and 13 weeks 6 days.

When will I start showing?
Usually between 12 and 16 weeks in a first pregnancy, and earlier in later pregnancies. At 10 weeks your uterus is still inside your pelvis, so anything visible is bloating.

Can I sleep on my stomach or back?
Yes. Any position is fine in the first trimester. Side-sleeping advice applies from around 28 weeks.

Is it safe to exercise?
Yes in an uncomplicated pregnancy, and it is recommended rather than merely allowed. Check first if you have bleeding, high blood pressure, a cervical stitch or any complication.

Is spotting normal in early pregnancy?
Light spotting happens in some pregnancies and is often harmless, but it should always be reported so it can be assessed.

Why am I so tired?
First trimester fatigue is hormonal and genuinely profound. It usually improves in the second trimester. Rest when you can, and get your haemoglobin checked, since anaemia is common.

I did not know I was pregnant and drank alcohol. What now?
Tell your doctor, stop now, and try not to dwell on it. Most women have a few weeks of not knowing, and one or two events in that window rarely change anything.

The bottom line

The first trimester is when your baby's major organs form and when you are likely to feel at your worst, which is an unfair combination. Two things genuinely change outcomes: taking folic acid and registering for antenatal care before 12 weeks, so the dating scan and NT scan happen in their correct windows. Beyond that, eat normally rather than for two, sleep however is comfortable, keep moving, and learn the four warning signs, because bleeding, severe one-sided pain, fainting and being unable to keep fluids down are all reasons to be seen rather than wait.

Sources

• World Health Organization, recommendations on antenatal care for a positive pregnancy experience

• American College of Obstetricians and Gynecologists, first trimester care, nutrition, exercise and early pregnancy loss

• NHS, your pregnancy week by week, vitamins and supplements in pregnancy, and foods to avoid

• National Institute for Health and Care Excellence, antenatal care guidance

• Fetal Medicine Foundation, nuchal translucency screening timing and crown-rump length criteria

• Indian Council of Medical Research and National Institute of Nutrition, nutrient requirements in pregnancy

• Ministry of Health and Family Welfare, Government of India, maternal health guidance, PMSMA, Anemia Mukt Bharat, and the PCPNDT Act

• Federation of Obstetric and Gynaecological Societies of India, antenatal care recommendations

This article is for general information and is not a substitute for medical advice. Seek care immediately for bleeding, severe one-sided abdominal pain, fainting, fever, or vomiting that prevents you keeping fluids down.

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