How big is your baby at 1 week?

There is no baby yet, and no pregnancy. At 1 week pregnant you are having your period. Pregnancy is counted from the first day of your last period, so the first two weeks of every pregnancy pass before an egg has even been released.

That is why this page gives no length and no size comparison. The first stage worth describing comes in week 3, once an egg has been fertilised.

Why does pregnancy start before you are pregnant?

Because the first day of a period is a date you can notice; the day an egg was fertilised is not. The American College of Obstetricians and Gynecologists (ACOG) sets out the convention: your due date is 280 days after the first day of your last period. That assumes a 28-day cycle with ovulation on day 14, so conception is taken to happen about two weeks into the count.

The convention has limits. ACOG notes that only about half of women accurately recall the date of their last period, and the calculation cannot allow for irregular cycles or variable ovulation. It calls a first-trimester ultrasound the most accurate way to date a pregnancy; in one study it cites, 40% of women given one had their due date adjusted because scan and period dates differed by more than 5 days.

Our due date calculator does the arithmetic and adjusts for a longer or shorter cycle.

What is happening in your body this week?

Your cycle is starting again. The NHS counts the first day of a period as day 1 of the menstrual cycle. Periods last around 2 to 7 days, with a typical loss of about 20 to 90 ml of blood — roughly 1 to 5 tablespoons.

Behind the bleeding, rising oestrogen prompts an ovary to develop an egg, and the lining of the womb starts to thicken again, ready for that egg to be released around the end of week 2.

That timing only fits a textbook cycle. The NHS says regular cycles from 21 to 35 days are normal and ovulation usually happens 10 to 16 days before the next period, so with a long cycle, conception comes later than the calendar assumes.

1 week pregnant symptoms: stomach pain, spotting and discharge

Anything you notice in week 1 comes from your period, not a pregnancy. There is no embryo and no pregnancy hormone yet. The NHS says the early signs of pregnancy usually start when you are around 4 to 6 weeks pregnant.

Stomach pain and cramps: period pain, which the NHS says happens when the womb tightens. It can spread to your back and thighs and usually lasts up to 3 days.
Spotting or bleeding: this is the period itself.
White discharge: vaginal secretions change through every cycle; around ovulation the NHS says they become thinner and stretchy, a bit like raw egg white. See a GP if your discharge changes in a way that concerns you.

One exception: if this "period" was much lighter or shorter than usual, it may not have been a period. The NHS describes implantation bleeding as a bleed like a very light period, with spotting or only a little blood, around the time your period is due. In that case you may have conceived last cycle and be nearer 4 weeks pregnant. Implantation bleeding vs a period sets out the differences; a test settles it.

Is 1 week pregnant the same as 1 week after sex?

No, and it is an easy mix-up to make. If you had unprotected sex a week ago, you are counting from conception, not from your period. On the dating doctors use, a pregnancy from that sex would be around 3 weeks along, not 1: see 3 weeks pregnant.

Either way, it is too early to test. Pregnancy tests detect hCG, which the NHS says starts to be produced around 6 days after fertilisation. The NHS advises testing from the first day of a missed period or, if you do not know when your period is due, at least 21 days after you last had unprotected sex. Our pregnancy test date calculator works out the day.

1 week pregnant belly, ultrasound and pictures

There is nothing to see, inside or out: your belly has not changed, and an ultrasound would show a womb and ovaries with no pregnancy in them. For when a bump really appears, see when do you start showing.

Is 1 week pregnant 1 month?

Week 1 is the first week of month 1. Month 1 runs from week 1 to week 4, ending around the time your next period would be due. There is no official weeks-to-months map, and a calendar month averages about 4.3 weeks rather than 4; our weeks to months converter shows how the common mappings line up.

If you are trying to conceive this cycle

The NHS advice on preparing for pregnancy applies now, not after a positive test:

Folic acid: 400 micrograms every day while you are trying, ideally from 3 months before, until 12 weeks of pregnancy. Some people need a higher, prescribed dose — see folic acid and prenatal vitamins.
Alcohol, smoking and drugs: the NHS lists not drinking alcohol, getting support to stop smoking and not using illegal drugs.
Medicines and conditions: do not stop a prescribed medicine before talking to your GP or specialist, and speak to a GP before trying if you live with a condition such as epilepsy or diabetes.
The date: write down the first day of this period. It is the starting point for your due date, and memory is unreliable.

Our ovulation and fertile window calculator estimates when this cycle's fertile days are likely to fall.

When to see a GP about your period

This is the NHS advice in the UK; elsewhere, contact your own doctor or local health line.

Ask for an urgent GP appointment or contact NHS 111

Pelvic pain or period pain that is severe or worse than usual, and painkillers have not helped

See a GP

Your periods become more painful, heavier or irregular, or the pain stops you doing your usual activities
You bleed between periods, or have pain during sex or when peeing or pooing

If you could already be pregnant

If a test has been positive, or this bleed was not like your usual period, the NHS ectopic pregnancy advice applies. Contact your GP or NHS 111 if you have a combination of a missed period, tummy pain low down on one side, vaginal bleeding or a brown watery discharge, pain in the tip of your shoulder, or discomfort when peeing or pooing — even if you have not had a positive test.

Call 999 in the UK, or your local emergency number, for a sharp, sudden and intense pain in your tummy with feeling very dizzy or fainting, feeling sick or looking very pale. The NHS says these can mean a fallopian tube has ruptured, which needs surgery as soon as possible.

Frequently asked questions

Can you have pregnancy symptoms at 1 week?

No. At 1 week you are on your period and not yet pregnant, so there is no pregnancy hormone to cause symptoms. Cramps and spotting this week come from the period itself. The NHS says early signs of pregnancy usually start at around 4 to 6 weeks pregnant, and not everyone has them.

Can a pregnancy test be positive at 1 week pregnant?

Not for a pregnancy from this cycle. Tests detect hCG, which the NHS says is produced from around 6 days after fertilisation, and fertilisation is still about two weeks away. A positive result this week points to a pregnancy from an earlier cycle, so talk to a GP or midwife about the bleed you took for a period.

How many weeks pregnant am I if I conceived a week ago?

About 3 weeks, on the standard dating. Doctors count from the first day of your last period and assume conception about two weeks after that, so one week after conception is roughly 3 weeks pregnant. A test will not be reliable until around the day your period is due, and an early scan confirms the dates.

1 week pregnant