Pregnancy Due Date Calculator

How to Use the Pregnancy Due Date Calculator

1. Open the pregnancy due date calculator.
2. Enter the first day of your last menstrual period (LMP). This is the standard clinical starting point — it is counted as Day 1 of pregnancy even though conception has not occurred yet.
3. Enter your average cycle length if it differs from the standard 28 days — the calculator adjusts the ovulation date accordingly.
4. Results show your estimated due date, current gestational age, and weeks remaining.
5. Alternatively, enter a known or estimated conception date for a directly conception-based EDD.

Due Date Calculation Methods

Pregnancy Milestones by Week

Why 40 Weeks from Last Period?

Pregnancy is conventionally measured from the first day of the last menstrual period (LMP), not from conception. This means the first 2 weeks of "pregnancy" occur before conception — during the menstrual and follicular phases. Actual embryonic development begins around week 3 (after ovulation and fertilisation).

The 40-week convention persists because:

• LMP is reliably known (conception date usually is not)
• Clinical databases, growth charts, and screening schedules all use LMP-based gestational age
• 40 weeks from LMP corresponds to 38 weeks from conception for a typical 28-day cycle
• The convention predates home ovulation testing and ultrasound — establishing a consistent reference point mattered practically

Gestational Age vs. Fetal Age

Two different ages are used to describe pregnancy:

• Gestational age: Counted from the first day of LMP. This is what healthcare providers use — "you are 12 weeks pregnant" means 12 weeks since LMP.
• Fetal age (embryonic age): Counted from conception — approximately 2 weeks less than gestational age for a typical cycle. Used in embryology research but rarely in clinical practice.
• Post-menstrual age: Used for premature babies after birth — their age is still counted from the original EDD to determine developmental expectations.

How Accurate Is a Due Date?

Only about 5% of babies are born on their exact estimated due date. Birth is considered "on time" between 39 and 40 weeks. The typical range is 37–42 weeks:

• 37–38 weeks: early term
• 39–40 weeks: full term (ideal)
• 41 weeks: late term
• 42+ weeks: post-term (in UK, induction typically offered at 41–42 weeks)

Factors affecting timing: first-time mothers tend to deliver slightly later on average; fetal sex (boys slightly later than girls on average); genetics (maternal birth timing is somewhat hereditary); and clinical induction for post-dates pregnancies.

NHS Antenatal Schedule (UK)

Key NHS appointments and timeline:

• Booking appointment (8–10 weeks): First midwife appointment; blood tests, blood pressure, weight; lifestyle advice; referral for scans
• Dating scan (10–14 weeks): Confirms EDD via crown-rump length; checks for twins
• Combined screening (11–14 weeks): Nuchal translucency scan + blood tests for Down's, Edwards', Patau's syndromes
• Anatomy scan (18–21 weeks): Detailed structural check of baby; checks placenta position
• Midwife appointments: At 16, 25, 28, 31, 34, 36, 38, 40, 41 weeks
• Blood tests at 28 weeks: Full blood count, antibodies check; anti-D injection if Rhesus negative

Common Questions

What if my cycle is not 28 days?

The standard calculation assumes a 28-day cycle with ovulation on Day 14. If your cycle is longer or shorter, ovulation shifts accordingly: a 30-day cycle means ovulation around Day 16, pushing conception and the EDD 2 days later than the standard calculation. The calculator adjusts for your cycle length — enter your actual average cycle length for a more accurate EDD. The first trimester ultrasound (dating scan) provides the most accurate EDD regardless of cycle length.

My NHS dating scan gave a different EDD than the calculator — which is correct?

Use the ultrasound EDD from your dating scan. Ultrasound measurement of crown-rump length (CRL) in the first trimester is accurate to ±5–7 days and is the clinical gold standard. The NHS policy is: if the scan-based EDD differs from the LMP-based EDD by more than 7 days, the scan date takes precedence. This is because LMP-based calculation assumes regular cycles and known period dates, which are not always reliable.

What does "gestational age" mean on my scan report?

Gestational age on a scan report means the estimated number of weeks and days of pregnancy from LMP — not from conception. A scan reporting "GA 12+3" means 12 weeks and 3 days gestational age. The equivalent fetal (embryonic) age is approximately 10 weeks 3 days. All antenatal screening results, growth percentiles, and clinical decisions are based on gestational age — this is why it is the universal clinical standard.