Pregnancy Due Date Calculator — EDD, Gestational Age & Milestones
One of the first questions in any pregnancy is: "When is my baby due?" This pregnancy due date calculator provides an estimated due date (EDD) using the three methods used in clinical practice: the Last Menstrual Period (LMP) method, the conception date method, and ultrasound-based correction. Alongside the EDD, it shows your current gestational age, pregnancy progress, trimester, and a table of key developmental milestones.
How Pregnancy Dating Works
Gestational age is measured in weeks from the first day of the last menstrual period (LMP), not from the date of conception. This convention is used universally in clinical obstetrics because the LMP is a reliably known date, whereas the precise moment of fertilisation is usually unknown. A full-term pregnancy is 40 weeks (280 days) by this convention, though only about 5% of babies are born on the exact due date. The majority of births occur within a two-week window centred on the EDD.
Three broad approaches are used to establish the EDD. Each has different assumptions and accuracy characteristics:
Method 1: Last Menstrual Period (Naegele's Rule)
Naegele's Rule is the standard clinical formula introduced by the German obstetrician Franz Karl Naegele in the early 19th century. In its classic form:
This formula assumes a 28-day menstrual cycle with ovulation occurring on day 14. This calculator extends Naegele's Rule with a cycle length adjustment:
For a woman with a 32-day cycle, the EDD is shifted four days later than the classic formula would indicate, because ovulation — and therefore fertilisation — occurs later in the cycle. For a 25-day cycle, the EDD is moved three days earlier. The luteal phase length is used to estimate the conception date shown in the results but does not directly change the EDD in the LMP method.
Method 2: Conception Date
When the approximate date of ovulation and fertilisation is known — for example, after IVF, intrauterine insemination (IUI), or confirmed OPK testing — the conception date method is more direct:
The figure of 266 days represents the typical gestational period from fertilisation to birth. Because ovulation normally occurs around day 14 of a 28-day cycle, an LMP-based EDD (adding 280 days) and a conception-based EDD (adding 266 days) should agree for a standard cycle. They diverge for irregular cycles or when the actual ovulation date differs from the calendar average. For IVF pregnancies, the clinic typically provides its own EDD based on the embryo age at transfer; this calculator can be used to cross-check that date.
Method 3: Ultrasound Correction
First-trimester ultrasound measurement of crown-rump length (CRL) is the most accurate method for dating a pregnancy, with a margin of error of approximately ±5 days at 6–10 weeks. The formula applied here is:
For example, if an ultrasound performed on a given date measures the pregnancy at 10 weeks 3 days (73 days), the EDD is that date plus (280 − 73) = 207 days. Ultrasound becomes progressively less precise for dating as pregnancy advances; after 20 weeks, the measurement window widens to ±2–3 weeks and is primarily used to monitor growth rather than to set or revise the EDD.
Trimesters and What They Mean
Pregnancy is traditionally divided into three trimesters, each representing a distinct phase of fetal development and maternal experience:
First trimester (weeks 1–13) — The embryonic period. All major organ systems form during these weeks. The risk of miscarriage is highest in the first trimester and falls significantly after week 10. Common symptoms include nausea, fatigue, and breast tenderness.
Second trimester (weeks 14–27) — Often called the "honeymoon trimester." Nausea usually eases, energy returns, and the pregnancy becomes visually apparent. The anatomy scan (typically at 18–21 weeks) checks fetal growth and organ development. Many parents learn the baby's sex during this scan.
Third trimester (weeks 28–40) — The baby gains most of its weight and prepares for life outside the womb. Prenatal appointments become more frequent. The baby is considered full term from 37 weeks.
Key Developmental Milestones
Several milestones carry particular clinical and personal significance:
Week 6 — Heartbeat detectable: Cardiac activity can be seen on transvaginal ultrasound from around 6 weeks. A visible heartbeat is an important reassurance milestone.
Week 8–10 — First prenatal scan: Many healthcare providers perform a viability and dating scan during this window. The CRL measurement at this stage provides the most accurate EDD.
Week 13 — End of first trimester: Miscarriage risk drops substantially. Non-invasive prenatal testing (NIPT) for chromosomal conditions is typically offered around this time.
Week 20 — Anatomy scan: The mid-pregnancy ultrasound checks for structural abnormalities and fetal growth. This is usually when fetal sex can be confirmed if desired.
Week 24 — Viability: From 24 weeks, a baby born prematurely has a medically recognised chance of survival with intensive care. Survival rates increase with each additional week.
Week 37 — Full term: The lungs are considered mature and the baby can be safely delivered. Delivery between 37 and 40 weeks 6 days is classified as full term.
Understanding Pregnancy Progress
The progress percentage shown in this calculator represents the proportion of a 40-week term that has elapsed since the LMP-equivalent date. It is a simple visual reference; it should not be read as an indicator of whether your pregnancy is "on track" — fetal development follows its own timeline, and the due date is an estimate, not a deadline.
Days remaining until the EDD is shown as a countdown. If today is past the EDD, the calculator shows how many days past the due date have elapsed. Most clinicians will discuss induction options when a pregnancy reaches 41–42 weeks, as the risk profile begins to shift after 40 weeks.
Limitations and Important Notes
All formulas used here are mathematical models derived from population averages. Individual pregnancies vary considerably. A few points to keep in mind:
Cycle length adjustment improves the LMP method but does not account for month-to-month variability in ovulation timing.
Ultrasound accuracy decreases as gestational age increases. First-trimester CRL measurements are significantly more reliable for dating than second-trimester biometrics.
The due date may be revised by your healthcare team at any point based on clinical findings. An ultrasound-based revision that differs by more than 5–10 days from the LMP date is typically adopted.
Multiple pregnancies (twins, triplets) typically deliver earlier than singleton pregnancies. This calculator is designed for singleton pregnancies.
This calculator is provided for informational and educational purposes. All pregnancy decisions — including care planning, testing, and delivery timing — should be made in consultation with a qualified healthcare provider.