Pregnancy Due Date Calculator
Calculate your due date, current pregnancy week, trimester, and conception estimate based on your last period or known due date.
How pregnancy dating works
Pregnancy is measured from the first day of the last menstrual period (LMP), not from the date of conception. This may seem counterintuitive — conception occurs roughly two weeks after LMP — but the LMP date is known with certainty, while the exact conception date usually is not. Obstetricians and midwives universally use LMP-based dating, making it the standard reference point throughout pregnancy care.
A full-term pregnancy lasts approximately 40 weeks, or 280 days, from LMP. This is calculated using Naegele's rule, a formula developed in the early 19th century that remains clinically standard today. The rule assumes a regular 28-day menstrual cycle with ovulation on day 14 — if your cycle is significantly longer or shorter, your estimated due date (EDD) may be adjusted after an early ultrasound.
Naegele's rule:
Due date = LMP + 280 days (40 weeks)
= LMP + 9 months + 7 days
Conception estimate:
Conception ≈ LMP + 14 days (assuming 28-day cycle)The three trimesters
Pregnancy is divided into three trimesters, each with distinct developmental milestones and common symptoms.
First trimester (weeks 1–12)
The first trimester encompasses the most rapid and critical phase of embryonic development. By week 10, all major organ systems have formed — the embryo is now called a foetus. The placenta takes over hormone production from the corpus luteum around week 10–12. Common symptoms include nausea, vomiting (morning sickness, though it can occur at any time), breast tenderness, fatigue, and frequent urination. Miscarriage risk is highest during this period, declining significantly after week 12.
Key appointments and tests: dating scan (8–12 weeks), combined first-trimester screening for chromosomal conditions (weeks 11–13), and blood tests to check blood type, iron levels, immunity to rubella, and sexually transmitted infections.
Second trimester (weeks 13–26)
For many people, the second trimester is the most comfortable phase of pregnancy. Nausea typically subsides, energy returns, and the pregnancy becomes visibly apparent. The foetus grows from about 7 cm and 43 g at week 13 to roughly 35 cm and 760 g at week 26. By week 18–20, foetal movements (quickening) become perceptible. Viability — the ability to survive outside the womb with intensive medical support — is generally considered to begin around week 22–24.
Key appointments: anomaly scan (18–21 weeks), which checks foetal anatomy in detail; glucose challenge test (24–28 weeks) to screen for gestational diabetes.
Third trimester (weeks 27–40)
The foetus gains most of its birth weight during the third trimester, growing from approximately 900 g at week 27 to a typical 3.1–3.6 kg at term. The brain undergoes rapid development, and the lungs mature in preparation for breathing. Common symptoms include back pain, heartburn, shortness of breath (as the uterus pushes against the diaphragm), Braxton Hicks contractions, and difficulty sleeping. The foetus typically moves into a head-down position by week 36.
Key appointments: growth scans if indicated, Group B Streptococcus (GBS) swab test (35–37 weeks), and increasing frequency of antenatal check-ups (weekly from week 36).
Week-by-week milestones
| Week | Size (approx.) | Key milestone |
|---|---|---|
| 4 | Poppy seed (1 mm) | Implantation occurs; hCG hormone triggers positive pregnancy test |
| 6 | Lentil (6 mm) | Heartbeat detectable on transvaginal ultrasound |
| 8 | Raspberry (1.6 cm) | All major organs begin forming; fingers and toes visible |
| 10 | Prune (3 cm) | Embryo officially becomes a foetus; tail disappears |
| 12 | Lime (5 cm) | End of first trimester; miscarriage risk decreases significantly |
| 16 | Avocado (12 cm) | Sex may be visible on ultrasound; skeleton begins hardening |
| 20 | Banana (25 cm) | Anomaly scan; foetal movements felt clearly |
| 24 | Corn (30 cm) | Viability threshold; lung development accelerates |
| 28 | Aubergine (35 cm) | Third trimester begins; eyes open and close |
| 32 | Squash (42 cm) | Foetus practices breathing movements; fat deposits accumulate |
| 36 | Honeydew (47 cm) | Considered early-term; head usually engaged in pelvis |
| 40 | Watermelon (~51 cm) | Full-term — due date! |
How accurate is the due date?
Only about 4% of babies are born on their exact due date. A normal, full-term birth can occur anywhere between 37 weeks 0 days and 41 weeks 6 days. The due date is best understood as the midpoint of a normal distribution centred around 40 weeks, with most spontaneous births occurring within 2 weeks either side.
| Gestational age | Classification | Proportion of births |
|---|---|---|
| < 28 weeks | Extreme preterm | ~0.5% |
| 28–31 weeks | Very preterm | ~0.7% |
| 32–36 weeks | Preterm / late preterm | ~10% |
| 37–38 weeks | Early term | ~26% |
| 39–40 weeks | Full term (optimal) | ~46% |
| 41–42 weeks | Late term | ~17% |
| > 42 weeks | Post-term | <1% (usually induced) |
Early ultrasound (performed at 8–13 weeks) is more accurate than LMP-based dating alone. When the ultrasound measurement differs from the LMP date by more than 7 days, most clinical guidelines recommend adjusting the EDD to match the ultrasound.
Factors that affect due date accuracy
Essential nutrients during pregnancy
| Nutrient | Daily recommendation | Why it matters | Sources |
|---|---|---|---|
| Folic acid | 400 mcg (pre-conception to week 12) | Prevents neural tube defects (spina bifida) | Supplement + leafy greens, fortified cereal |
| Iron | 27 mg/day | Supports increased blood volume; prevents anaemia | Red meat, legumes, fortified foods + vitamin C |
| Calcium | 1,000 mg/day | Foetal bone development; protects maternal bone density | Dairy, tofu, fortified plant milks, almonds |
| Omega-3 (DHA) | 200–300 mg DHA/day | Foetal brain and eye development | Oily fish (2 portions/week), algae supplement |
| Iodine | 220 mcg/day | Foetal thyroid development and brain growth | Dairy, seafood, iodised salt |
| Vitamin D | 600–2000 IU/day | Bone health; immune function; low levels linked to preeclampsia | Sunlight, oily fish, fortified foods, supplement |
Warning signs to know
Seek medical advice promptly if you experience any of the following during pregnancy:
- Bleeding: Any vaginal bleeding after a positive pregnancy test warrants assessment, though spotting is common in early pregnancy.
- Severe abdominal pain: Persistent, cramping, or one-sided pain could indicate ectopic pregnancy or placental complications.
- Reduced foetal movement: After week 24, contact your midwife if you notice a significant reduction in movements. Do not use at-home dopplers to reassure yourself — only professional assessment is reliable.
- Signs of pre-eclampsia: Sudden swelling of face, hands or feet; severe headache; visual disturbances; upper abdominal pain — this is a medical emergency.
- Symptoms of preterm labour: Regular contractions before 37 weeks, lower backache, or a feeling of pressure in the pelvis.
Frequently asked questions
My cycle is not 28 days — is the due date still accurate?
If your cycle is longer than 28 days, you likely ovulate later and your due date may be pushed forward. An early ultrasound (8–12 weeks) will give a more accurate estimate. If your cycle is shorter, the reverse applies. This calculator uses standard 28-day assumptions, as does Naegele's rule.
Can I calculate my due date after IVF?
For IVF pregnancies, gestational age is calculated from the egg retrieval date plus 14 days (for fresh transfers) or from the embryo transfer date plus the number of days of development (e.g. day 3 or day 5 embryo). The "due date" mode on this calculator accepts your confirmed due date if provided by your clinic.
What is a sweep and when does it happen?
A membrane sweep (stretch and sweep) is offered from 39–41 weeks to try to bring on labour naturally. A midwife or doctor sweeps a finger around the cervix to separate the membranes from the uterine wall, releasing prostaglandins. It is not guaranteed to work but reduces the likelihood of formal induction.
What happens if I go past my due date?
Going overdue by a few days is very common. In most countries, formal induction of labour is offered around 41–42 weeks because the risk of placental dysfunction and stillbirth begins to rise after 42 weeks. Your midwife or obstetrician will discuss the evidence and options with you.
Due date = Last Menstrual Period (LMP) + 280 days (Naegele's rule) Trimester 1: weeks 1–12 Trimester 2: weeks 13–26 Trimester 3: weeks 27–40 Conception is estimated at LMP + 14 days (assuming a 28-day cycle).