How to use the Pregnancy Calculator
- Choose the date you know: your due date, last period, ultrasound, conception date or IVF transfer.
- Enter the date and the extra details, such as your cycle length, the weeks shown on your scan or the embryo age.
- Press the Calculate button.
- See your due date, how many weeks pregnant you are today and which trimester you are in.
- Scroll through the milestone table to plan scans, tests and visits.
Pregnancy Calculator Guide: Pregnancy Length, Due Date and Healthy Habits
In short: a normal pregnancy lasts about 40 weeks from the first day of your last period, or about 38 weeks from conception. Your due date is an estimate. Only a small share of babies arrive on that exact day, but most are born within two weeks of it.
How Long Does Pregnancy Last?
Pregnancy is the time, roughly nine months, during which a baby develops inside the womb. Birth usually takes place about 38 weeks after conception, which is about 40 weeks counting from the first day of your last menstrual period (LMP). The World Health Organization treats a normal pregnancy length as anything from 37 to 42 weeks.
Your doctor or midwife will usually give you an estimated due date at your first prenatal visit, often based on an early ultrasound. The date can also be worked out from your last period, using our due date calculator, or from your conception date, using our conception calculator.
How Accurate Is a Due Date?
The due date is a good guide, but the real length of a pregnancy varies. Research on natural variation in pregnancy length suggests that:
- fewer than 4 in 100 babies are born on the exact due date,
- about 6 in 10 are born within a week of it,
- and almost 9 in 10 are born within two weeks of it.
Age, the length of earlier pregnancies and the mother’s weight can all affect timing, and other causes are still not fully understood. So you can plan around your due date, but nobody can predict the exact day of birth. For more detail, read how accurate due date calculators are and why your due date can change after an ultrasound.
Signs of Pregnancy and How Pregnancy Is Detected
Early signs
Many women first notice:
- a missed period,
- tiredness,
- nausea,
- needing to urinate more often,
- and a slightly higher resting body temperature if they chart it.
These signs can also have other causes, so a test is the only way to be sure.
Pregnancy tests
Pregnancy tests look for the hormone hCG, which the body makes only during pregnancy. A blood test ordered by a doctor can pick up very small amounts of hCG and may detect pregnancy about six to eight days after fertilisation. It is the most sensitive option, but it takes longer to get results and costs more than a home test.
Home urine tests are convenient and, used from the first day of a missed period, are reliable for most women. A urine test done in a clinic is not necessarily more accurate than a good home test. If you test early and get a negative result, test again after a few days. If you are unsure when you ovulated, our ovulation calculator can help you pick a sensible day.
Looking After Yourself During Pregnancy
Pregnancy care depends heavily on your own health and circumstances. The four areas below come up most often. In every case, your doctor, midwife or dietitian knows your situation best.
Medication
Some medicines can affect a developing baby. In the past, the US Food and Drug Administration sorted drugs into letter categories A, B, C, D and X, from lowest to highest fetal risk. That letter system has since been replaced by detailed written labelling, but the principle is unchanged: never start, stop or change a medicine, including herbal products and supplements, without asking your doctor or pharmacist first.
Weight gain
Gaining weight is a normal and necessary part of pregnancy. It supports the baby, placenta, extra blood and fluid, and your own energy stores. Gaining too little or too much can raise the risk of problems such as gestational hypertension and the need for a cesarean birth.
The Institute of Medicine suggests these total gains for a single pregnancy, based on your BMI before pregnancy:
| Pre-pregnancy BMI | Category | Suggested total gain |
|---|---|---|
| Below 18.5 | Underweight | 28 to 40 lb (about 12.5 to 18 kg) |
| 18.5 to 24.9 | Normal weight | 25 to 35 lb (about 11.5 to 16 kg) |
| 25 to 29.9 | Overweight | 15 to 25 lb (about 7 to 11.5 kg) |
| 30 or above | Obese | 11 to 20 lb (about 5 to 9 kg) |
These are general ranges and your own target may differ. Not sure of your starting category? Check it with our BMI calculator.
Exercise
Research shows that regular aerobic and strength exercise during pregnancy helps maintain fitness and may lower the chance of a cesarean birth. The American College of Obstetricians and Gynecologists says that in an uncomplicated pregnancy, exercise is unlikely to harm the baby. Women who exercised regularly before pregnancy can usually keep going, with their doctor’s approval.
Stop exercising and call your doctor if you notice any of the following:
- vaginal bleeding,
- shortness of breath before you start exercising,
- dizziness or headache,
- calf pain or swelling,
- chest pain,
- muscle weakness,
- leaking fluid,
- regular contractions or signs of early labour,
- or less movement from your baby.
Nutrition
Your body needs extra energy and more of certain nutrients during pregnancy. Two are especially well known:
- Folic acid (vitamin B9) lowers the risk of neural tube defects. Doctors commonly advise a daily supplement before conception and in early pregnancy.
- DHA (an omega-3 fat) supports the baby’s brain and eye development. It reaches the baby through the placenta and later through breast milk, and you can get it from food such as oily fish or from a supplement your doctor recommends.
Advice about what to eat and avoid can be confusing and may differ from person to person, so ask your doctor or a registered dietitian for a plan that suits you.
Plan Your Pregnancy Week by Week
Once you know your due date, you can follow your pregnancy by trimester and book scans and checkups at the right time. Use the tools on our site to find your dates, then take the results to your first prenatal appointment so your provider can confirm them.
Sources
Information in this guide draws on guidance from the World Health Organization, the American College of Obstetricians and Gynecologists, the Institute of Medicine, and published research on the natural variation in human pregnancy length (Jukic et al., 2013).
Disclaimer: This article is for general information and education only. It is not medical advice and does not replace consultation with a qualified doctor, midwife or other healthcare professional. Due dates, test timing, weight-gain ranges and other figures are estimates and may not apply to your situation.
Always speak to your healthcare provider about your own pregnancy, medicines, diet and exercise. If you have bleeding, severe pain, reduced baby movements or any other worrying symptom, contact your doctor or emergency services straight away.
Pregnancy Calculator FAQs
How many weeks is a full-term pregnancy?
A pregnancy lasts about 40 weeks from the first day of your last period. Full term is 39 weeks 0 days to 40 weeks 6 days. Babies born from 37 weeks are considered early term.
Why is pregnancy 40 weeks if conception happens around week 2?
Doctors count from the first day of your last period because it is usually known. Conception happens about 2 weeks later, so the baby grows for around 38 weeks.
What are the three trimesters?
The first trimester runs to the end of week 13. The second runs from week 14 to the end of week 27. The third runs from week 28 until birth.
When is the anatomy scan?
Most providers book the detailed anatomy scan between 18 and 22 weeks.
When is glucose screening done?
Screening for gestational diabetes is usually done between 24 and 28 weeks, earlier if you have risk factors.
Are the milestone dates the same for everyone?
No. They are general guides based on your dates. Your doctor or midwife will plan the checks that suit you.