You can do most pregnancy tests from the first day of a missed period, and the NHS says they are most reliable from then (NHS). If you do not know when your period is due, the NHS says to test at least 21 days after you last had unprotected sex (NHS).
When can I take a pregnancy test?
The NHS (the UK’s National Health Service) says all pregnancy tests look for the hormone hCG. It starts to be produced around 6 days after fertilisation, when an egg and a sperm join (NHS). A positive result is almost certainly correct, and a negative one is less reliable (NHS). A result may not be reliable if you test too early or do not follow the instructions (NHS). Some very sensitive tests can be used even before you miss a period (NHS).
The calculator adds your cycle length to the first day of your last period, as the Period Calc app does. That gives the day your next period is due. We use the day your period is due as the first day you can call it missed. The NHS does not give an exact day. If your period has not arrived by the end of it, the advice above applies.
The formula
Day shown = first day of the last period + cycle length (the day your next period is due)
Days to wait = day shown − today
Days late = today − day shown, once it has passed
A worked example
Your last period started on , and your cycle is 30 days long.
- Next period due
- ( + 30 days)
- If today is
- 5 days to wait
- If today is
- 4 days late: your period was due on
- In the Hijri calendar
- last period on , period due
What if my period is late?
Here, “late” only means later than your usual cycle length says. When today is past the due day, the calculator gives the day your period was expected and how many days ago. Pregnancy is one of the normal causes of a missed period (ACOG, the American College of Obstetricians and Gynecologists). If your period starts, enter its first day instead. For the common reasons and when to see a doctor, read why a period can be late.
A bleed similar to a very light period, with some spotting or only a little blood, around the time your period is due is known as implantation bleeding. The NHS lists it as one possible early sign of pregnancy (NHS).
What if the test is negative?
If you get a negative result and still think you are pregnant, the NHS says to wait a few days and try again (NHS). It says to speak to your GP (family doctor) if you get a negative result after a second test but your period has not arrived (NHS). If your period is late and the test is negative, read what a negative test can mean.
What if my cycles are irregular?
The NHS says it is difficult to pinpoint exactly when ovulation happens. It says the time from ovulation to the next period is around 10 to 16 days (NHS). So the day the calculator gives can be off. If you do not know when your next period is due, the NHS says to test at least 21 days after you last had unprotected sex (NHS). For what counts as irregular, read irregular periods.
When to see a doctor
If a test is positive, tell a doctor or midwife soon. In England the NHS says it is important to tell the NHS as soon as possible after you find out you are pregnant (NHS). To see how many weeks pregnant you are, use the pregnancy week calculator.
An ectopic pregnancy is when a fertilised egg implants outside the womb, usually in a fallopian tube. It can be serious, so the NHS says to get advice right away. Do this if you might be pregnant and have a combination of the symptoms below, even without a positive test. In the UK, contact your GP (family doctor) or call NHS 111 (NHS):
- a missed period and other signs of pregnancy
- tummy pain low down on one side
- vaginal bleeding or a brown watery discharge
- pain in the tip of your shoulder
- discomfort when peeing or pooing
In the UK, the NHS says to call 999 or go to A&E (accident and emergency) immediately if you have a combination of (NHS):
- a sharp, sudden and intense pain in your tummy
- feeling very dizzy or fainting
- feeling sick
- looking very pale
Outside the UK, use your local emergency number. ACOG says to go to an emergency room if you have sudden, severe pain, shoulder pain or weakness (ACOG).
See a doctor if your period stops for about 3 months without a reason (ACOG). If you had IVF (in vitro fertilisation), your clinic gives you a pregnancy test and a date to use it, around 16 days after the embryo transfer (NHS). Follow that date, not this calculator.
Estimates only, not medical advice. No test result is promised on any day.
Questions about taking a pregnancy test
Can I take a pregnancy test before my period is due?
Some very sensitive tests can be used even before you miss a period (NHS). But the NHS says tests are most reliable from the first day of a missed period, and a test taken too early may not be reliable (NHS). If you get a negative result and still think you are pregnant, the NHS says to wait a few days and try again (NHS).
My test is negative but my period has not come. What now?
The NHS says to wait a few days and try again. It says to speak to your GP (family doctor) if you get a negative result after a second test but your period has not arrived (NHS).
How accurate are home pregnancy tests?
The NHS says home tests are accurate as long as you follow the instructions correctly. It says a positive result is almost certainly correct while a negative one is less reliable (NHS).
Does the contraceptive pill stop a pregnancy test working?
No. The NHS says the hormones in the contraceptive pill will not stop a pregnancy test working, although some medicines can affect the result (NHS).
How sure is the due day if my cycles are irregular?
Not very. The calculator adds your usual cycle length to the first day of your last period, so it is only as good as your cycle is regular. If you do not know when your next period is due, the NHS says to test at least 21 days after unprotected sex (NHS).
How this page was made
Hossam Hamouda, the developer of Period Calc, wrote this page from the guidance of the health organisations listed under Sources. They have not reviewed it, and neither has a doctor. The calculator runs the formula above in your browser, with the same rules as the Period Calc app. It works from averages, so it cannot know how your own body differs from them, and it cannot diagnose anything: if something worries you, see when to see a doctor.
Sources
- Doing a pregnancy test (NHS)
- Periods and fertility in the menstrual cycle (NHS)
- Signs and symptoms that might mean you're pregnant (NHS)
- What to do when you find out you're pregnant (NHS)
- Ectopic pregnancy (NHS)
- IVF (NHS)
- Ectopic Pregnancy (ACOG)
- Amenorrhea: Absence of Periods (ACOG)