Pregnancy Calculator
Calculate due date, conception date, and key pregnancy milestones.
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About
Pregnancy Calculator
Pregnancy describes the roughly nine months during which one or more offspring develops inside a woman. Birth usually happens about 38 weeks after conception, or about 40 weeks after the last menstrual period. The World Health Organization defines a normal term as 37 to 42 weeks.
At a first appointment an OB-GYN normally gives an estimated due date, based on a scan or on the last menstrual period. This calculator builds the same schedule from whichever date you already have.
How a due date is worked out
Every method here resolves to one anchor, the first day of the last menstrual period, and everything else follows from it.
Due date = last menstrual period + 280 days
That is Naegele's rule, dating from the early nineteenth century, and it assumes a 28-day cycle with ovulation on day 14. The calculator applies the same 280 days no matter which input you choose; the five modes differ only in how they get back to the anchor.
| If you know | How the anchor is found |
|---|---|
| Due date | Last period = due date minus 280 days |
| First day of last period | Used directly, shifted by any difference between your cycle length and 28 days |
| Ultrasound date and gestational age | Last period = scan date minus the measured weeks and days |
| Conception date | Last period = conception minus 14 days |
| IVF transfer date | Last period = transfer minus 14 days minus the embryo age |
Cycle length matters because ovulation, not menstruation, starts the clock. On a 32-day cycle ovulation is around day 18 rather than day 14, so the due date moves four days later. The calculator adjusts for this whenever you enter a cycle other than 28 days.
IVF dating is the most precise of the five, because the date of fertilisation is known rather than estimated. A day-5 blastocyst transferred on 7 March gives a due date of 23 November: the embryo is already five days past fertilisation, so the notional last period sits 19 days before the transfer.
Gestational age against fetal age
Pregnancy is counted from the last period, not from conception, which means you are already counted as two weeks pregnant at the moment of conception. Gestational age is therefore about two weeks ahead of the actual age of the fetus. Weeks 1 and 2 on the schedule above happen before there is anything to be pregnant with.
This trips people up constantly. When a scan reports "9 weeks", that is gestational age, and the embryo is around 7 weeks old. Every clinical milestone, every screening window and every date in the table below uses gestational age.
How close is the due date, really?
Fewer than 4% of births happen on the exact due date. About 60% arrive within a week of it, and almost 90% within two weeks. So you can be reasonably confident of a fortnight-wide window and no more.
Actual length varies with the mother's age, the length of her previous pregnancies and her own birth weight, and a good deal of the variation is not understood at all. A 2013 study by Jukic and colleagues, which tracked ovulation directly rather than relying on recalled dates, found natural variation in pregnancy length spanning 37 days once preterm births were excluded. That is over five weeks of genuine biological spread around a single predicted date.
Ultrasound dating in the first trimester is the most accurate method available, measuring crown-rump length to within about five to seven days. Accuracy falls off as pregnancy progresses, because babies of the same age vary more in size the further along they get, which is why an early scan is used to set the dates and later scans are not.
What the term labels mean
ACOG replaced the single idea of "term" with four bands in 2013, because outcomes at 37 weeks and 39 weeks are not the same.
| Label | Gestational age |
|---|---|
| Preterm | before 37 weeks |
| Early term | 37 weeks 0 days to 38 weeks 6 days |
| Full term | 39 weeks 0 days to 40 weeks 6 days |
| Late term | 41 weeks 0 days to 41 weeks 6 days |
| Post term | 42 weeks and beyond |
Detecting pregnancy
Pregnancy can be detected by a test, or noticed through symptoms: a missed period, a raised basal body temperature, fatigue, nausea and needing to urinate more often.
Tests detect hormones that act as biomarkers, principally hCG, which is only present during pregnancy. Clinical blood and urine tests can pick it up six to eight days after fertilisation. Blood tests are more accurate and can measure the exact amount of hCG earlier and at lower concentrations, but they take longer and cost more than a home urine test. A clinical urine test is not necessarily more accurate than a home one, and can cost more.
Home tests are most reliable from the day of a missed period. Testing earlier produces false negatives, because hCG doubles roughly every 48 hours in early pregnancy and simply has not built up enough yet.
Managing a pregnancy
Medication
Some medicines taken during pregnancy have lasting effects on the fetus. The FDA used to sort drugs into categories A, B, C, D and X by benefit against fetal risk, with A being low risk and X meaning proven risk that outweighs any benefit. Those letters were retired under the Pregnancy and Lactation Labeling Rule, which began phasing in from 2015 and replaced them with narrative summaries of the actual evidence, since a single letter hid too much. Older references still use the letters. Either way, discuss anything you take, including over-the-counter medicines and supplements, with your doctor.
Weight gain
Weight gain in pregnancy is necessary and unavoidable, and it varies between people. It covers the baby, the placenta, extra circulating fluid, and stores of fat and protein. Both too little and too much carry risks, including caesarean delivery and gestational hypertension. The Institute of Medicine recommends gains based on pre-pregnancy BMI.
| Pre-pregnancy BMI | Recommended gain |
|---|---|
| Under 18.5, underweight | 28 to 40 lb |
| 18.5 to 24.9, normal | 25 to 35 lb |
| 25 to 29.9, overweight | 15 to 25 lb |
| 30 and above, obese | 11 to 20 lb |
Most of that arrives in the second and third trimesters. The old advice about eating for two does not survive contact with the numbers: the extra requirement is roughly 340 calories a day in the second trimester and 450 in the third, which is a sandwich rather than a second dinner.
Exercise
Aerobic exercise during pregnancy helps maintain fitness and may reduce the chance of a caesarean. Regular aerobic and strength work is generally recommended, and women who trained hard before pregnancy and have an uncomplicated one can usually continue at high intensity. ACOG suggests that in an uncomplicated pregnancy, exercise is unlikely to injure the fetus, and recommends at least 150 minutes of moderate activity a week.
Stop and speak to a doctor if you get vaginal bleeding, shortness of breath before exertion, dizziness, headache, calf pain or swelling, amniotic fluid leakage, reduced fetal movement, signs of preterm labour, muscle weakness or chest pain.
Nutrition
Nutrition matters more than usual here, because energy and micronutrient requirements both change. Folic acid, vitamin B9, lowers the risk of neural tube defects, and the recommendation is 400 micrograms a day starting at least a month before conception, since the neural tube closes by around week four, often before anyone knows they are pregnant. DHA, an omega-3 fatty acid needed for brain and retinal development, cannot be made efficiently by infants and reaches them only through the placenta or later in breast milk.
Iron requirements roughly double, because blood volume rises by 40% to 50%. Many other micronutrients matter too, and the advice available online is endless and contradictory. Take the specifics from your doctor or a dietitian, who can work from your own situation.
What this calculator cannot do
The schedule assumes a single pregnancy. Twins and other multiples run differently and are typically delivered earlier, so the dates here do not transfer. The average sizes come from population data and healthy babies vary widely around them, in both directions, without anything being wrong.
None of this replaces antenatal care. A due date set by an early scan overrides one calculated from a remembered period date, and your clinician's dating is the one that counts.
Common questions
Frequently asked questions
Add 280 days, or 40 weeks, to the first day of the last menstrual period. This is the rule set out by Naegele, and it assumes a 28-day cycle with ovulation on day 14. Every other method here works backwards to that same anchor first.
Because the last period is a date most people can name, while conception usually is not. The side effect is that you are already counted as two weeks pregnant at the moment of conception, so gestational age runs about two weeks ahead of the actual age of the fetus.
Fewer than 4% of babies arrive on the exact date. About 60% come within a week of it and almost 90% within two weeks. A 2013 study by Jukic and colleagues that tracked ovulation directly found natural variation spanning 37 days, so treat the date as the centre of a fortnight-wide window.
Yes. Ovulation starts the clock, not menstruation. On a 32-day cycle ovulation happens around day 18 rather than day 14, which pushes the due date four days later. Enter your average cycle length and the calculator adjusts for it.
It is more precise, because the date of fertilisation is known rather than estimated. Subtract 14 days plus the embryo age from the transfer date to get the notional last period. A day-5 blastocyst transferred on 7 March gives a due date of 23 November.
A first-trimester ultrasound, which measures crown-rump length and dates the pregnancy to within about five to seven days. Accuracy falls later on, because babies of the same age differ more in size as pregnancy progresses. That is why an early scan sets the dates and later scans do not override them.
ACOG splits it into bands: early term is 37 weeks 0 days to 38 weeks 6 days, full term is 39 weeks 0 days to 40 weeks 6 days, late term is 41 weeks, and 42 weeks onwards is post term. The distinction exists because outcomes at 37 and 39 weeks are not the same.
Clinical blood and urine tests can find hCG six to eight days after fertilisation. Home urine tests are most reliable from the day of a missed period, because hCG roughly doubles every 48 hours and needs time to build up. Testing early is the usual cause of a false negative.
The Institute of Medicine bases it on pre-pregnancy BMI: 28 to 40 lb if underweight, 25 to 35 lb at a normal BMI, 15 to 25 lb if overweight and 11 to 20 lb if obese. The extra energy needed is around 340 calories a day in the second trimester and 450 in the third.
No. The schedule assumes a single pregnancy. Multiples grow differently and are usually delivered earlier, so both the size figures and the dates will be wrong. Use dating from your own antenatal care instead.