Pregnancy Week 1: What's Happening in Your Body

11 min read

Quick answer: In the UK, pregnancy week 1 starts on the first day of your last period, so conception hasn't happened yet and you won't have pregnancy symptoms. Your uterus sheds its lining, FSH rises and follicles begin to mature. If you're trying to conceive, start 400 micrograms of folic acid daily and 10 micrograms of vitamin D, stop smoking, avoid alcohol, eat well and review any medications with your GP.

What parents need to know

If you've just found out you're pregnant or are trying to conceive, the idea of "pregnancy week 1" can be a little confusing. That's because, in the UK, pregnancy is measured from the first day of your last menstrual period (LMP), not from the moment of conception. So technically, during week 1, you're not yet pregnant in the biological sense. But your body is already preparing for everything that comes next.

Here's everything you need to know about what's happening during week 1 of pregnancy, and how to set yourself up for the healthiest possible start.

Pregnancy dating	From the first day of your last period
Folic acid	400mcg daily until end of week 12
Vitamin D	10mcg daily supplement
Due date	40 weeks from first day of last period
Dating scan	Usually offered between 10 and 14 weeks

How is pregnancy dated in the UK?

The NHS dates pregnancy from the first day of your last period. This means your 40-week pregnancy begins before conception has taken place. It might seem counterintuitive, but this system exists because ovulation (and therefore conception) can be difficult to pinpoint precisely, whereas your period is usually easier to track.

During week 1, your uterus is shedding its lining from your previous cycle. Your body is, in effect, resetting and preparing a fresh environment for a potential pregnancy.

What's happening in your body in week 1?

While there's no embryo yet, your body is far from idle:

Menstruation: Your uterus is shedding the endometrial lining from the last cycle.
Hormonal shifts: FSH (follicle-stimulating hormone) levels begin to rise, signalling your ovaries to prepare to release an egg.
Follicle development: Several follicles in your ovaries begin to mature, each containing a potential egg.
The uterine lining rebuilds: After menstruation ends, oestrogen kicks in and a fresh lining begins to form.

You won't feel any pregnancy symptoms this week because there's no pregnancy yet. Any symptoms you experience are simply those of a normal period.

Your due date

Once you confirm a pregnancy, your midwife or GP will calculate your estimated due date (EDD) by counting 40 weeks from the first day of your last period. You can also use the NHS Pregnancy Due Date Calculator to get an estimate.

Your dating scan (usually offered between 10 and 14 weeks) will confirm your due date more precisely.

What to do in week 1: preconception advice

If you're trying to conceive, week 1 of your cycle is the perfect time to lay the groundwork. The NHS recommends:

Start taking folic acid

The NHS advises all women trying to conceive to take a daily supplement of 400 micrograms (mcg) of folic acid. This should be taken from before conception until the end of the 12th week of pregnancy. Folic acid significantly reduces the risk of neural tube defects such as spina bifida.

Some women need a higher dose (5mg), which a GP can prescribe. The NHS recommends it if you or the baby's father has a neural tube defect or a relative with one, you have had a previous pregnancy affected by a neural tube defect, you have diabetes, you have sickle cell anaemia or thalassaemia, or you take medicine for epilepsy or HIV. The Royal College of Obstetricians and Gynaecologists (RCOG) also advises the higher dose if you have a BMI of 30 or more. If any of these apply to you, ask your GP whether you need the higher dose.

Take a vitamin D supplement

If you're pregnant, the NHS recommends taking 10 micrograms (mcg) of vitamin D every day between early October and late March. Take it all year if you usually cover most of your skin when you're outside or spend a lot of time indoors, and consider taking it all year if you have black or brown skin. Vitamin D helps your baby's bones and teeth develop.

Stop smoking

Smoking is linked to an increased risk of miscarriage, premature birth, low birth weight, and stillbirth. If you smoke, now is the best time to stop. The NHS Stop Smoking service offers free support and has helped millions of people quit successfully.

Reduce alcohol

There is no proven safe level of alcohol in pregnancy. The NHS advises that if you are pregnant or trying to get pregnant, the safest approach is not to drink alcohol at all.

Eat a balanced diet

You don't need to eat for two, but eating a varied, nutritious diet will help your body prepare for pregnancy. Focus on:

Plenty of fruits and vegetables (at least 5 portions a day)
Wholegrains, lean proteins, and dairy or dairy alternatives
Foods rich in iron, calcium, and iodine
Oily fish (no more than 2 portions per week)

Avoid foods that carry a risk of listeria or salmonella, including unpasteurised cheeses, raw or undercooked eggs, and pâté. Do not take cod liver oil or supplements containing vitamin A (sometimes called retinol), and avoid liver and liver products, because too much vitamin A can harm your baby's development. Always check the label.

Review any medications

If you or your partner take any prescription or over-the-counter medicines, or any herbal remedies, speak to a GP, ideally before you start trying for a baby. Do not stop taking a prescribed medicine until you've talked to your GP or specialist, who can discuss the safest treatment option for you while you're trying to get pregnant. Before taking any medicine, including painkillers, check with your pharmacist, midwife or GP that it's suitable, and tell them you are pregnant or trying for a baby.

Should you see a GP before getting pregnant?

A preconception appointment is a brilliant idea. Your GP can review your medical history, check your rubella immunity status, and advise on any health conditions that may need to be managed before or during pregnancy. This is especially important if you have diabetes, epilepsy, thyroid conditions, or a history of mental health difficulties.

Tracking your cycle

Understanding your cycle can help you identify your fertile window. Most people ovulate around day 14 of a 28-day cycle, but this varies widely. Signs of ovulation include:

A slight rise in basal body temperature (BBT)
Changes in cervical mucus (becoming clearer and more stretchy, like egg white)
A mild twinge or pain on one side of the lower abdomen (known as Mittelschmerz)

Ovulation predictor kits (OPKs), available from pharmacies, detect the LH surge that precedes ovulation and can help you time intercourse more effectively.

Managing stress and wellbeing

Trying to conceive can be emotionally demanding. Taking care of your mental health is just as important as physical preparation. Regular exercise, adequate sleep, and connecting with supportive friends or your partner can all make a real difference. If you're feeling overwhelmed, speaking to your GP is always a good first step.

When to get help

Some symptoms in pregnancy can be a sign of something serious and need checking quickly. This is who to contact, and how soon.

Call 999 if:

you're bleeding from your vagina and the bleeding is heavy (soaking a period pad soon after putting it on), or you also have severe pain in your tummy, pain in your shoulder, or feel sick, faint, dizzy or lose consciousness
you have a sharp, sudden and intense pain in your tummy together with feeling very dizzy or fainting, feeling sick or looking very pale, as this could mean an ectopic pregnancy has ruptured (you can also go straight to A&E)
you have severe difficulty breathing (you're gasping, choking or not able to get words out) or your chest feels tight or heavy
you have pain and swelling in one leg and you also feel short of breath or have chest pain. A blood clot in the leg can travel to your lungs, which can be life-threatening
you have possible signs of sepsis, such as breathing very fast, being confused or having slurred speech, blue, pale or blotchy skin, a rash that does not fade when you press it, or a very high or very low temperature. Being pregnant makes sepsis more likely
you do not feel you can keep yourself safe (a mental health emergency is as serious as a physical one)

Always call 999 if you think it's a life-threatening emergency, no matter what stage of pregnancy you are.

Contact your GP or early pregnancy unit right away if:

you have light bleeding or spotting, even if you have no pain or only mild tummy pain
you have vaginal discharge that's unusual for you, or fluid leaking from your vagina
you have tummy pain low down on one side, or pain in the tip of your shoulder (where your shoulder ends and your arm begins). Symptoms of an ectopic pregnancy usually start between 4 and 12 weeks, so get advice right away, even if you have not had a positive pregnancy test yet
you have stomach pain that is severe or does not go away after you've rested for 30 to 60 minutes, or stomach pain with regular cramping or tightenings, lower back pain, or pain when you pee
your breathlessness comes on suddenly, you're out of breath when you're lying down, or you're breathless and also have chest pain or feel dizzy
you have pain or swelling in one leg, usually in the calf or thigh. This could be a blood clot (deep vein thrombosis, or DVT), which is more likely when you're pregnant
you have a high temperature (38C or above), or you feel hotter than usual or shivery. A high temperature can be a sign of an infection, so it's important to get it checked

If you cannot reach them, or you're not sure what to do, call NHS 111.

Speak to your GP or midwife if:

you're worried about your mental health, or stress, anxiety or low mood are affecting your day-to-day life. If you need urgent help for your mental health, call 111 and select the mental health option, or ask for an urgent GP appointment

Key takeaways

Week 1 in the NHS system is the first day of your last period. Conception hasn't happened yet
Start folic acid (400mcg/day) and vitamin D now if you're trying to conceive
Stop smoking and avoid alcohol
Eat well, review medications, and consider a preconception GP appointment
Track your cycle to identify your fertile window

Frequently asked questions

Are you actually pregnant in week 1?

No. Because pregnancy is dated from the first day of your last period, conception hasn't happened yet in week 1, so you aren't pregnant in the biological sense and won't have pregnancy symptoms.

How much folic acid should I take when trying to conceive?

The NHS advises 400 micrograms of folic acid daily, from before conception until the end of week 12. Some women need a higher dose of 5mg, for example if there is a family history of neural tube defects, or you have diabetes or take medicine for epilepsy. The RCOG also advises it if you have a BMI of 30 or more. Ask your GP whether you need the higher dose.

Is any alcohol safe when trying to get pregnant?