How due dates are calculated - Naegele's rule
The most widely used method, endorsed by the American College of Obstetricians and Gynecologists (ACOG Committee Opinion #700, 2017, reaffirmed 2022), is Naegele's rule: estimated due date (EDD) = first day of the last menstrual period (LMP) + 280 days (40 weeks). The rule assumes a 28-day cycle with ovulation on day 14; our calculator adjusts for shorter or longer cycles by adding (cycle_length - 28) days. For first-trimester ultrasound dating (more accurate after 8 weeks of pregnancy), your healthcare provider may revise the EDD based on fetal measurements.
The three trimesters
Pregnancy is divided into three trimesters by ACOG convention. The 1st trimester runs from week 0 (LMP) through 13 weeks 6 days - characterized by major organ formation, early symptoms (nausea, fatigue), and the first prenatal visits. The 2nd trimester (14 weeks 0 days - 27 weeks 6 days) is often called the 'golden trimester': symptoms ease, energy returns, the baby's movements begin to be felt, and the anatomy scan is performed. The 3rd trimester (28+ weeks) brings rapid fetal growth, more frequent prenatal visits, and preparation for delivery.
Key pregnancy milestones
End of 1st trimester (13 weeks): risk of miscarriage drops significantly, many parents share the news publicly. Anatomy scan window (18-22 weeks, typically week 20): detailed ultrasound to check fetal anatomy and often reveal sex. Viability threshold (24 weeks): infants born at this point have a chance of survival with intensive neonatal care. Full term begins (37 weeks 0 days): WHO and ACOG classify births from this point as full-term. Estimated due date (40 weeks): only about 4% of babies are born on this exact date. Post-term watch (42+ weeks): your provider will typically discuss induction options.
Tracking pregnancy in the Hijri calendar
Many families in the Arab world track key life events - including pregnancy and birth - on the Hijri calendar alongside the Gregorian. The pregnancy itself remains 280 days regardless of which calendar you use; only the labels change. Our calculator shows the due date in both calendars so you can plan around religious observances (Ramadan, Hajj) and discuss timing with both your healthcare provider (Gregorian) and your family records (Hijri). A 40-week pregnancy spans approximately 9-10 Hijri months given the lunar/solar difference.
Accuracy and limitations
Naegele's rule gives a reasonable estimate but has known limitations. About 4% of pregnancies deliver on the exact predicted date; ~66% deliver within ±7 days; ~95% within ±14 days. First-trimester ultrasound dating (weeks 7-13 6 days) is more accurate than LMP and may be used to revise the EDD - particularly for irregular cycles, when LMP is uncertain, or when LMP-based and ultrasound-based dates differ by more than 7 days in the first trimester. Always follow your obstetrician's clinical dating over this calculator's estimate.
What to do next
Once you have an estimated due date, the standard prenatal care path includes: confirming pregnancy with your healthcare provider (typical first visit at 8-10 weeks), starting prenatal vitamins (folic acid 400-800 mcg daily, plus iron and calcium as recommended), scheduling routine antenatal visits (monthly through week 28, then every 2 weeks until week 36, then weekly until delivery, per WHO Antenatal Care recommendations), and beginning the conversations about birth preferences. In Saudi Arabia, the Ministry of Health offers free antenatal care at primary healthcare centers for citizens and residents.
Frequently asked questions
It's a reasonable estimate based on Naegele's rule, but only about 4% of babies are born on the exact predicted date. About 66% of births fall within ±7 days, and ~95% within ±14 days. First-trimester ultrasound dating (between weeks 7 and 13+6) is more accurate and is what your healthcare provider will use to refine the date.
Use your best estimate - even a date within ±3 days will give a workable result. If you have irregular cycles or can't remember at all, your healthcare provider can use a first-trimester ultrasound (8-13 weeks) which measures the fetus and gives a more accurate due date than LMP. The ACOG guideline explicitly recommends switching to ultrasound dating if first-trimester scan differs from LMP by more than 7 days.
Yes - enter your average cycle length in the second field. Naegele's rule assumes a 28-day cycle with ovulation on day 14; we adjust by adding (your cycle - 28) days. So a 32-day cycle pushes the due date 4 days later; a 25-day cycle pulls it 3 days earlier.
By medical convention, week 1 of pregnancy starts on the first day of your last menstrual period - which is actually about 2 weeks before conception in a typical 28-day cycle. This is because LMP is a date most women can remember, while conception is often uncertain. So when you're '8 weeks pregnant,' the embryo is biologically about 6 weeks old.
Yes - switch the toggle to Hijri. Pregnancy is 280 days regardless of which calendar you use; we just convert to and from Hijri so you can enter your LMP in whichever calendar you track. A 40-week pregnancy spans ~9-10 Hijri months given the lunar/solar difference. Many Saudi and other Arab families track both calendars for family records and religious observances (Ramadan, Hajj).
Sources
- Committee Opinion #700 - Methods for Estimating the Due Date— American College of Obstetricians and Gynecologists (ACOG)
- WHO recommendations on antenatal care for a positive pregnancy experience— World Health Organization (WHO)
- Antenatal care for uncomplicated pregnancies - NICE Guideline NG201— National Institute for Health and Care Excellence (NICE), UK
- Maternal Health Services - Saudi Arabia— Ministry of Health, Kingdom of Saudi Arabia
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