The first trimester: what happens

The first 12 weeks of pregnancy

The first trimester is a period of remarkable change, when all of the baby's vital organs form. It is at the same time a period of hormonal upheaval for the mother — which is why it often turns out to be the physically hardest while nothing is yet visible from the outside.

About the weeks, first. Dating is counted not from conception but from the first day of the last period. So by the time a test shows two lines the "date" is usually already around 4–5 weeks, even though fertilisation happened roughly two weeks after the start of the count. This is not an error — it is the standard way of counting, and every marker below works on it.

What is happening with the baby

• around week 6: the heart begins to beat. On a scan it can usually be seen from week 6–7, and the exact timing varies — a little earlier or later does not mean anything is wrong.
• week 8: fingers, ears, eyes and a face are forming.
• week 10: the main organs are laid down; from here they grow and mature.
• week 12: the baby is already moving arms and legs, and the kidneys are working.

The weeks here are a guide, not a timetable: development varies slightly, and exact dating is determined by a clinician from a scan.

The main thing worth knowing about this period: organs are laid down in these first weeks, which is why questions about medicines, supplements and harmful exposures matter more sharply in the first trimester than later. That is not a reason for retrospective panic — it is a reason to put them to a clinician at the very first appointment.

What is happening with the mother

The hormonal changes of the first trimester — rising hCG and progesterone — coincide with the familiar symptoms:

• Nausea: most often in the morning, but it can last all day. It usually starts around week 6 and settles for many people by the end of the trimester.
• Increased tiredness: the body is spending a great deal of energy building a placenta. This is not "weakness" and not a reason to push yourself.
• Tender breasts: preparation for lactation, often one of the very first signs.
• Passing urine more often: blood volume and the work your kidneys are doing both rise; pressure from the uterus comes later.
• Changes in smell and taste: a sudden aversion to familiar smells is common.
• Emotional swings: a consequence of hormonal change, multiplied by tiredness.

An absence of symptoms is also within the range of normal. Some people feel almost nothing, and that does not mean anything is wrong. Equally, symptoms disappearing towards the end of the trimester most often just means the hormonal peak has passed.

What usually helps with nausea

This is not treatment but the everyday measures most commonly suggested:

• eat smaller amounts more often than usual, and do not let yourself get very hungry;
• keep something dry by the bed and eat it before getting up;
• drink small amounts often rather than a lot at once;
• avoid the smells and foods that set it off for you specifically — the list is different for everyone;
• ginger in various forms helps many people, but it is not a universal answer.

If nausea is stopping you drinking or eating at all, that is no longer an everyday matter but a reason to contact a clinician. More on that below.

What is fine and what is better avoided

The most common first-trimester questions — with honest answers, marked where a general guide stops.

• Coffee. Guidance usually suggests limiting caffeine rather than cutting it out entirely. The specific limit is worth confirming at an appointment.
• Alcohol. Guidance names no safe level, so the general recommendation is to stop.
• Exercise. Moderate activity is usually not contraindicated and is in fact good for you. Sharply increasing load, contact sports and anything with a fall risk are a separate conversation with a clinician.
• Medicines. This is the main point. Some routine drugs — including ones sold without a prescription — are not used during pregnancy. Check before taking, not after, and not against a description found online.
• Food. Guidance names specific categories to avoid: raw or undercooked meat and fish, unpasteurised dairy, raw eggs, some fish high in mercury, unwashed greens.
• Hair dye, nails, flying. Questions almost everyone has, with no universal answer — they depend on your dating and your situation. That is a perfectly normal thing to ask at an appointment.

The first appointment and what happens at it

Booking in is usually advised during the first trimester. The first appointment is longer than the ones that follow, because it is where the full picture is collected.

What you will usually be asked:

• the first day of your last period;
• your usual cycle length;
• previous pregnancies and how they went;
• chronic conditions and regular medication;
• inherited conditions and birth defects in the family.

The first two determine the estimated dating, and this is exactly where records in an app save time: the date does not have to be recalled. If your cycle is longer or shorter than 28 days, that is worth mentioning too — the calculation changes because of it.

First-trimester screening is usually offered between about week 11 and week 14. What it includes and which tests you need is determined by a clinician.

What almost everyone worries about

This is rarely written about directly, although almost everyone thinks about it.

The risk of pregnancy loss is highest in the first trimester, and about half of those losses are down to random chromosomal events; in the rest, no cause is usually identified at all. Either way it is not down to anything anyone did. Which means ordinary work, moderate exercise, stress at your job or a cup of coffee drunk before the pregnancy was known about are not the cause.

Knowing that is useful not in order to stop worrying, but in order not to spend your strength looking for your own fault where there is none.

On anxiety that does not settle, and when it stops being ordinary worry, see Anxiety during pregnancy.

When to contact a clinician urgently

Contact your midwife, GP or NHS 111 straight away if you have:

1. 1.Vaginal bleeding.
2. 2.Pain low in the abdomen or in one side.
3. 3.Vomiting after which you cannot keep fluids down — not even water. What matters here is not how many times, but that you cannot drink.
4. 4.A temperature of 38 °C or above.
5. 5.Sudden dizziness or fainting.

Severe one-sided pain together with dizziness or fainting is the combination that does not wait until morning: that is how an ectopic pregnancy can present, and the number for it is 999 (or 112).

Contact your midwife or GP the same day if pain or burning appears when you pass urine. A urinary infection in pregnancy is not left untreated, but it is a reason for an appointment rather than for an ambulance.

What is worth recording

The first trimester is a period in which a lot of things happen for the first time, and memory is not built for it.

• the first day of your last period (dating is counted from it);
• how you feel day by day: nausea, tiredness, appetite;
• everything you take, supplements included;
• questions that come up between appointments — there are always more of them than you remember once you are actually there;
• mood.

The pregnancy journal in Kvit is encrypted like everything else: the key stays on the phone. What that means technically is in Is it safe to track your cycle in an app.