How to Calculate Pregnancy Weeks: A Step-by-Step Guide from LMP to Due Date

Gestational age forms the basis of many decisions during pregnancy — the timing of screening tests, ultrasound measurements, the estimated due date, and the assessment of fetal growth all depend on it. In this article I explain step by step how pregnancy weeks are calculated, which methods are most reliable, and why the due date is given as a range rather than a single day.

Gestational age is counted from the last menstrual period

Obstetric convention counts pregnancy from the first day of the last menstrual period (LMP), not from the date of conception. The reason is practical: the LMP is a date most women can remember, whereas fertilisation typically occurs around 2 weeks later and is much harder to date. As a result gestational age is about 2 weeks longer than fetal age (the time since conception).

Example: if the first day of your LMP was 8 weeks ago, you are 8 weeks pregnant. The embryo itself is around 6 weeks old.

Calculating the estimated due date (Naegele's rule)

The most widely used formula is Naegele's rule. Apply the following to the first day of your LMP:

• Add 7 days.
• Subtract 3 months.
• Add 1 year.

The result is the estimated due date (EDD) at 40 weeks (280 days). For example, if your LMP was 10 January 2026, your EDD is 17 October 2026.

When cycles are irregular

Naegele's rule assumes a 28-day cycle with ovulation on day 14. When cycles are irregular or considerably longer, the LMP-based calculation may be several days off. In this situation ultrasound dating in the first trimester is far more accurate.

Ultrasound dating and when the due date is revised

During the first trimester the fetus grows in a very consistent way. Measuring the crown–rump length (CRL) between weeks 7 and 13 provides accurate dating to within a few days. Current obstetric guidelines redate the pregnancy from the ultrasound when:

• The difference from the LMP is more than 5 days between 6–8 weeks.
• The difference is more than 7 days between 9–13 weeks.

After the first trimester, biparietal diameter, femur length, and abdominal circumference are used together for dating and growth assessment; however, dating becomes progressively less precise as the pregnancy advances.

IVF pregnancies

Because the exact fertilisation and embryo transfer dates are known, IVF pregnancies are dated directly from those dates rather than the LMP. For example, for a day-5 embryo transfer, gestational age at transfer is 2 weeks and 5 days. IVF dating is the most precise form of pregnancy dating available.

The trimesters

• First trimester (weeks 1–13): Embryonic development, formation of the major organs. Screening tests and early ultrasound are performed in this window.
• Second trimester (weeks 14–27): Detailed anatomy scan (18–22 weeks); this is generally the most comfortable phase of pregnancy, and fetal movements are typically first felt around 18–20 weeks.
• Third trimester (weeks 28–40): Rapid fetal growth, gestational diabetes and blood pressure screening, and preparation for delivery.

Term and post-term pregnancy

Only about 5% of babies are born on the calculated due date; delivery typically occurs within a two-week window around that date:

• Early term: 37 weeks 0 days – 38 weeks 6 days
• Full term: 39 weeks 0 days – 40 weeks 6 days
• Late term: 41 weeks 0 days – 41 weeks 6 days
• Post-term: 42 weeks 0 days and later

Beyond 41 weeks the risks to the fetus increase, and induction of labour is generally considered.

Common questions

Can I calculate the due date if my cycle is irregular? The LMP-based calculation may be inaccurate. Ultrasound dating in the first trimester gives the most reliable answer.

Is the calculation different in twin pregnancies? The dating principle is the same, but delivery usually occurs earlier (around 36–37 weeks for twins).

Does the due date change during the pregnancy? If a significant discrepancy is found in the first trimester, the due date can be revised. After the first trimester the initial date is maintained.

Pregnancy care in Adana

Accurate dating from the first weeks makes it possible to plan screening tests and ultrasound at the right time and to spot problems early. For personalised, evidence-based pregnancy care in Adana, you can contact Op. Dr. Sadık Kükrer.