Pregnancy Calculator، Accurate Due Date and Current Week

Two questions dominate early pregnancy: when is the due date? and what week am I in right now? This tool answers both to clinical accuracy and adjusts for your actual cycle length.

How the calculator works

The tool applies Naegele's rule، the standard used across obstetric practice since the 19th century:

Estimated due date = first day of LMP + 280 days (40 weeks)

Sources: ACOG، Methods for Estimating the Due Date · WHO، Antenatal Care.

Correction for irregular cycles

The original rule assumes a 28-day cycle. In reality, normal cycles range from 21 to 35 days. The calculator adjusts automatically:

• 30-day cycle → adds 2 days to the due date
• 26-day cycle → subtracts 2 days
• and so on

What the calculator tells you

| Output | Formula | |---|---| | Estimated due date | LMP + 280 days + cycle correction | | Current gestational week | (today − LMP) ÷ 7 | | Current trimester | First: 1–13, Second: 14–27, Third: 28–40 | | Days remaining until birth | Due date − today | | Approximate conception date | LMP + 14 days (ovulation day) |

Gestational weeks in detail

First trimester (weeks 1–13)

| Week | Key event | |---|---| | 1–2 | Ovulation and fertilization | | 3 | Implantation in the uterus | | 4–6 | Neural tube and heart begin to form | | 7–9 | Limbs and facial features develop | | 10–13 | Primary organ formation completes |

Common symptoms: morning sickness, fatigue, breast tenderness, frequent urination.

Health warning: avoid medications without medical advice, and avoid smoking, alcohol, and X-rays.

Second trimester (weeks 14–27)

| Week | Key event | |---|---| | 14–16 | Facial features sharpen; organs mature | | 17–20 | Fetal sex becomes visible on ultrasound | | 20 | First movements the mother can feel | | 24–27 | Senses develop (especially hearing) |

Common symptoms: rising energy, better appetite, visible bump, fetal movement.

Important screenings: gestational diabetes test (between weeks 24 and 28).

Third trimester (weeks 28–40)

| Week | Key event | |---|---| | 28–32 | Rapid weight and brain growth | | 33–36 | Lungs mature in preparation for breathing | | 37 | Pregnancy is considered "full term" | | 38–40 | Fetus prepares for delivery |

Common symptoms: heaviness, Braxton Hicks contractions, milk leakage.

True labor signs: regular contractions every 5 minutes, rupture of membranes (water breaking), bloody show.

When to call your doctor immediately

• Vaginal bleeding at any stage
• Severe abdominal or back pain
• Severe headache with blurred vision
• Sudden swelling of the face or hands
• Absent fetal movement for more than 12 hours (in the second and third trimesters)
• Fever above 38°C
• Continuous vaginal fluid leakage (possible ruptured membranes)

These are warning signs، do not wait.

Calendar dating versus ultrasound

| Criterion | Calendar (LMP) | Ultrasound (first trimester) | |---|---|---| | Accuracy | ±2 weeks | ±5 days | | Availability | Immediately after pregnancy is confirmed | Between week 7 and 13 | | Requirement | Accurate LMP date | Clinical exam | | Reliable with irregular cycles | No | Yes |

Recommendation: use the calendar as an initial estimate and rely on ultrasound as the definitive reference.

Trusted sources

• ACOG، Methods for Estimating the Due Date (Committee Opinion 700)
• Harvard T.H. Chan، Nutrition During Pregnancy
• NIH، Pregnancy Health Info
• ACOG، American College of Obstetricians and Gynecologists
• WHO، Antenatal Care Guidelines

Medical disclaimer: This tool and article are informational only، not a substitute for medical advice. Consult an obstetrician-gynecologist for personalized care throughout your pregnancy.

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