How this pregnancy calculator works
Choose a starting point: due date (work backward to LMP and milestones), last menstrual period (Naegele: +280 days), ultrasound date plus gestational age from LMP, conception date (+266 days to due date), or IVF transfer with Day 3 or Day 5 embryo (common rules: +263 or +261 days). Output includes estimated LMP, due date, gestational age as of today, trimester, and a milestone table — for planning and education, not a substitute for your OB or midwife.
Gestational age vs. fertilization age
Gestational age (from LMP) is what most prenatal visits use: 40 weeks from the first day of the last period is full term. Fertilization age is about two weeks less when ovulation occurs around cycle day 14. Birth often occurs near 38 weeks after conception or 40 weeks after LMP. The WHO cites normal term roughly 37–42 weeks gestational age.
Naegele's rule and ultrasound dating
Naegele's rule estimates due date as LMP + 280 days, assuming a 28-day cycle. Irregular cycles make LMP-only dating less reliable. Crown–rump length on ultrasound in the first trimester (often ~8–13 weeks) is a standard way to confirm or adjust due dates. IVF due dates can be calculated precisely from transfer protocol because embryo age is known.
Pregnancy term and due date
Pregnancy describes the period in which a fetus develops until birth. Length varies by person and pregnancy. Studies commonly cite that only a small fraction of births happen exactly on the estimated due date, with most within one to two weeks. Your clinician integrates history, ultrasound, and exam — not an online calculator alone.
Pregnancy detection
Symptoms such as a missed period, nausea, breast tenderness, fatigue, or frequent urination may suggest pregnancy but are not definitive. Home and clinical tests detect hCG. Urine tests are convenient; quantitative blood tests can detect pregnancy very early and track levels when medically indicated.
Trimesters at a glance
- First (weeks 1–13): Organ formation, common early symptoms; prenatal care and risk discussions begin.
- Second (weeks 14–26): Often more energy; anatomy ultrasound and movement usually occur in this window.
- Third (weeks 27–40): Rapid fetal growth, birth planning, and monitoring for complications.
Pregnancy management — overview
Prenatal care cadence (typical)
Many schedules use monthly visits through 28 weeks, visits about every two weeks until 36 weeks, then weekly until delivery — adjusted for risk and local practice.
Medication (FDA pregnancy categories — historical context)
The former A/B/C/D/X letter system summarized animal and human data for fetal risk. Labeling has moved toward narrative risk summaries, but you may still see letters in older references. Always review prescriptions, OTC drugs, and supplements with your clinician.
Weight gain (Institute of Medicine, pre-pregnancy BMI)
| Pre-pregnancy BMI | Recommended total gain |
|---|---|
| Underweight (< 18.5) | 28–40 lb |
| Normal (18.5–24.9) | 25–35 lb |
| Overweight (25–29.9) | 15–25 lb |
| Obese (≥ 30) | 11–20 lb |
Too little or too much gain can carry risks; your provider may personalize targets. Our BMI Calculator can help classify pre-pregnancy BMI from height and weight — it is not a substitute for medical advice.
Exercise
Aerobic and strength work are often continued in uncomplicated pregnancies with modifications. Stop and seek care for warning signs such as bleeding, severe headache, calf pain or swelling, fluid leakage, decreased fetal movement, chest pain, or regular painful contractions before term.
Nutrition
Folate/folic acid reduces neural tube defect risk; iron needs rise; calcium and DHA support bone and neurologic development. Discuss prenatal vitamins, fish intake, caffeine limits, and food safety (e.g. unpasteurized products, high-mercury fish) with your team.
Sample milestone timeline (approximate)
- Weeks 1–4: conception and implantation window; early hCG rise
- ~Week 6: cardiac activity often visible on ultrasound
- ~Week 12: end of first trimester for many counseling purposes
- Weeks 18–22: anatomy survey common
- ~Week 24: intensive-care viability threshold in many settings
- Week 28: third trimester in common counting
- Week 37+: early/full term definitions per ACOG/WHO usage — confirm with your clinician
- Week 40: estimated due date from LMP-based dating
References and further reading
- Jukic AM, Baird DD, Weinberg CR, et al. Length of human pregnancy and contributors to its natural variation. Hum Reprod. 2013;28(10):2848–2855.
- Moore K. How accurate are 'due dates'? BBC News, 2015.
- Institute of Medicine. Weight Gain During Pregnancy: Reexamining the Guidelines. National Academies Press; 2009.
- Davies GA, Wolfe LA, Mottola MF, et al. Exercise in pregnancy and the postpartum period. J Obstet Gynaecol Can. 2003;25(6):516–529.
- Artal R, O'Toole M. ACOG guidelines for exercise during pregnancy and the postpartum period. Br J Sports Med. 2003;37(1):6–12.
- Lammi-Keefe CJ, Couch SC, Philipson E. Handbook of Nutrition and Pregnancy. Humana Press; 2008.