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Which pregnancy tool do you need?
- [Due date calculator](/due-date-calculator) – estimates your due date from the first day of your last period (adjusted for cycle length), the date of conception, an IVF embryo transfer, or an ultrasound scan. It shows how many weeks pregnant you are, your trimester and key milestones.
- [Ovulation calculator](/ovulation-calculator) – estimates your ovulation day and six-day fertile window for the next few cycles from your last period, cycle length and luteal phase, and shows the due date if you conceive in the current cycle.
How the two tools connect
Both calculators rest on the same timeline. Pregnancy is counted from the first day of the last menstrual period (LMP), about two weeks before conception in a 28-day cycle. That is why a pregnancy is "40 weeks" long even though the embryo is about 38 weeks old at the due date.
| Event | Day (28-day cycle) | Gestational age |
|---|---|---|
| First day of last period | Day 1 | 0 weeks |
| Ovulation and conception | About day 14 | 2 weeks |
| Missed period | About day 29 | 4 weeks |
| End of first trimester | – | 13 weeks 6 days |
| Full term begins | – | 39 weeks 0 days |
| Estimated due date | LMP + 280 days | 40 weeks 0 days |
If your cycle is longer or shorter than 28 days, ovulation – and therefore the due date – shifts by the same number of days. Both calculators let you enter your usual cycle length.
Which due date counts?
The American College of Obstetricians and Gynecologists (ACOG) recommends that an ultrasound in the first trimester is the most accurate way to establish or confirm the due date. If the scan and the period-based estimate differ by more than a set amount for the stage of pregnancy, the due date is changed to match the scan. Pregnancies conceived by IVF are dated from the embryo transfer. Once set, the due date should not normally be changed later in pregnancy.
Term definitions
ACOG defines the following categories:
| Category | Gestational age |
|---|---|
| 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 |
| Postterm | 42 weeks 0 days and beyond |
Only a small share of babies are born on their exact due date, so it helps to think of a window of several weeks rather than a single day.
The fertile window
The fertile window is the six days ending on the day of ovulation, based on the work of Wilcox and colleagues. Sperm can survive for several days, but the egg survives for only about a day after ovulation, which is why the days before ovulation matter most. The ovulation calculator explains how to combine its estimate with ovulation (LH) tests and cervical mucus changes, and when to seek advice if you have been trying to conceive.
Before and during pregnancy
The CDC recommends that anyone who could become pregnant takes 400 micrograms of folic acid every day, starting at least one month before pregnancy, to help prevent neural tube defects. These calculators do not give medical advice about pregnancy; your midwife, obstetrician or doctor will guide your care, and you should contact them promptly about bleeding, severe pain, reduced baby movements later in pregnancy, or any symptom that worries you.
Related tools
- BMI calculator – for before pregnancy; adult BMI categories do not apply during pregnancy.
- How to read a blood test – many routine pregnancy blood tests use pregnancy-specific ranges.
- TSH levels – including trimester-specific TSH ranges.