Pregnancy Week 1: What's Happening in Your Body

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.

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 actually 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 is 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. Here's what 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 may need a higher dose (5mg) — for example, if you have a BMI over 30, a family history of neural tube defects, or take certain medications. Speak to your GP to confirm the right dose for you.

Take a vitamin D supplement

The NHS recommends that all adults — including pregnant and breastfeeding women — take a daily supplement of 10 micrograms (mcg) of vitamin D. It supports bone health and immune function in both you and your baby.

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.

Review any medications

If you take any prescription or over-the-counter medications, speak with your GP about whether they are safe to continue during pregnancy. Some medications need to be swapped for pregnancy-safe alternatives before conception.

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.

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 5mg, for example with a BMI over 30 or a family history of neural tube defects. Ask your GP.

Is any alcohol safe when trying to get pregnant?

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.

When do you ovulate?

Most people ovulate around day 14 of a 28-day cycle, though this varies widely. Signs include a slight rise in basal body temperature, clearer stretchy cervical mucus, and a mild one-sided twinge known as Mittelschmerz.