How many weeks pregnant am I? Pregnancy is usually dated from the first day of your last menstrual period, not from the day of conception. If your cycle is regular, an NHS due date calculator can give an estimate. A dating scan can provide a more accurate estimate of how far along you are.
Pregnancy normally lasts around 37 to 42 weeks from the first day of the last period. A due date is an estimate, not a promise that labour will start on that date.
How many weeks pregnant am I? How pregnancy weeks are counted
Gestational age starts on day one of your last period. In a typical 28-day cycle, ovulation and conception often occur about two weeks later. This means the pregnancy count can include roughly two weeks before conception. If your cycles are longer, shorter or irregular, a calculator based on a 28-day cycle may be less precise.
For example, if the first day of your last period was 8 weeks ago, you may be described as around 8 weeks pregnant, even though conception likely happened later. Only a clinician or scan can estimate your dates more accurately.
How to estimate your due date
Use the first day of your last period in the NHS due date calculator. You may be able to adjust for a cycle that is shorter or longer than 28 days. If you do not remember the date, or your periods are irregular, contact your GP or midwife for advice rather than relying on a guess.
In England, a dating scan is usually offered at around 10 to 14 weeks. The sonographer measures the pregnancy and may adjust the estimated due date. Scan timing and maternity pathways can differ across the UK, so follow advice from your local maternity service.
What if I do not know my last period date?
That is common. Tell your midwife or GP what you do know, such as a rough date or whether your periods were regular. They can help arrange the right care. Do not delay contacting maternity services once you know you are pregnant; early antenatal care gives you time to discuss health, medicines and screening options.
If you have not yet confirmed pregnancy, a home test is most reliable from the first day of a missed period. If you do not know when your period is due, NHS guidance says to test at least 21 days after unprotected sex.
When to get urgent advice in pregnancy
Contact your maternity unit, GP or NHS 111 if you have concerning symptoms and are unsure what to do. Call 999 for a life-threatening emergency. The right service can depend on how many weeks pregnant you are and your local area.
For related reading, see our guides to NHS fertility testing, NHS-funded IVF in the UK and recovery in the first six weeks after birth.
Frequently asked questions
Are pregnancy weeks counted from conception?
No. Clinicians usually count from the first day of your last menstrual period. Conception often happens about two weeks later in a typical cycle.
How accurate is an online pregnancy due date calculator?
The calculator helps answer the question, “How many weeks pregnant am I?”, by giving an estimate from the dates you enter. A dating scan can provide a better estimate, particularly if your cycle is irregular or you are unsure of your last period date.
What is a dating scan in the UK?
It is an ultrasound scan used to estimate how far along a pregnancy is and when the baby may be due. Ask your local maternity team about timing in your area.
When should I contact a midwife?
Contact NHS maternity services as soon as you find out you are pregnant so antenatal care can be arranged.
Sources: NHS due date calculator, NHS 12-week scan, and NHS pregnancy tests. General information only.
