Pregnancy Due Date Calculator: Find Your Estimated Due Date
One of the first questions every expectant parent asks is: when is my baby due? Our Pregnancy Due Date Calculator uses Naegele's Rule — the global clinical standard — to estimate your due date (EDD) from the first day of your last menstrual period (LMP) or from your known conception date. The formula adds 280 days (40 weeks) to the LMP, giving you a baseline EDD. An early ultrasound remains the most accurate confirmation, but this calculator gives you a clinically sound estimate from day one.
Naegele's Rule. Assumes a 28-day cycle with ovulation on day 14. Adjust forward or backward if your cycle length differs. From conception date: EDD = Conception Date + 266 days (38 weeks).
Understanding Naegele's Rule
Naegele's Rule works because human pregnancy lasts approximately 40 weeks from the last menstrual period, which is roughly 38 weeks from conception (since conception typically occurs 2 weeks into the cycle). The rule is a practical convention, not a biological certainty. It assumes: (1) a 28-day menstrual cycle, (2) ovulation on day 14, and (3) fertilisation on that same day. If your cycle length differs, the calculator adjusts the EDD accordingly.
The Three Trimesters: What Happens When
First Trimester (Weeks 1–12): The embryo develops all major organ systems. Morning sickness and fatigue are common. The nuchal translucency scan at 11–13 weeks checks for chromosomal conditions. Second Trimester (Weeks 13–26): The 'honeymoon phase' — nausea typically improves, the bump becomes visible, and fetal movement (quickening) begins around 18–22 weeks. Third Trimester (Weeks 27–40): Rapid weight gain as the baby prepares for birth. Prenatal visits become more frequent toward the EDD.
How Ultrasound Can Revise Your Due Date
An early ultrasound — ideally between 6 and 12 weeks — measures the Crown-Rump Length (CRL) of the embryo. This is a direct measure of fetal size and is more accurate than LMP-based dating alone, especially if your cycle is irregular. If the ultrasound EDD differs from your LMP-based EDD by more than 7 days in the first trimester, your clinician will typically update the official due date to the ultrasound figure. In the second trimester, the accuracy window widens to approximately ±2 weeks.
Full Term, Preterm, and Post Term: What the Weeks Mean
Pregnancy duration is measured in weeks from the LMP. Preterm birth is before 37 weeks — associated with higher risks for the newborn. Early term is 37–38 weeks. Full term is 39–40 weeks, considered the optimal delivery window. Late term is 41 weeks and post term is 42+ weeks, at which point induction is usually discussed. Only about 4% of babies are born exactly on their due date.
Key Pregnancy Milestones by Week
- Week 6–8: Heartbeat detectable by ultrasound
- Week 12: End of first trimester, miscarriage risk drops significantly
- Week 20: Anatomy scan — major anomaly screening
- Week 28: Third trimester begins, Rh factor and glucose screening
- Week 37: 'Early term' — baby is considered viable
- Week 40: EDD — the expected due date
Signs That Labour Is Approaching
- Lightening: Baby drops lower into the pelvis
- Mucus plug loss: Thick discharge from the cervix
- Braxton Hicks increase: Practice contractions become more frequent
- Nesting instinct: Burst of energy and urge to prepare the home
- Water breaking: Rupture of membranes — call your midwife or doctor immediately
- Regular contractions: Increasing in frequency, duration, and intensity
Frequently Asked Questions
How accurate is the due date calculator?
This calculator gives a clinically standard estimate using Naegele's Rule. Only about 4% of babies are born on their exact due date. Most full-term births occur within a 2-week window around the EDD. An early ultrasound (before 14 weeks) is the most accurate way to confirm gestational age.
What is Naegele's Rule?
Naegele's Rule is the standard obstetric formula for estimating the due date. It adds 280 days (40 weeks) to the first day of your last menstrual period (LMP). This assumes a standard 28-day cycle with ovulation on day 14. It was developed by German obstetrician Franz Karl Naegele in the early 19th century and remains the clinical standard today.
What if my cycle is longer or shorter than 28 days?
If your cycle is longer (e.g., 35 days), ovulation likely occurs later, and your EDD should be adjusted forward by the extra days. If your cycle is shorter (e.g., 21 days), your EDD moves earlier. Our calculator accounts for cycle length to give a more personalised estimate.
Can an ultrasound change my due date?
Yes. An early ultrasound (ideally at 6–12 weeks) measures the crown-rump length (CRL) of the embryo and can confirm or revise the EDD. If the ultrasound date differs by more than 7 days from the LMP-based date in the first trimester, most clinicians will update the official due date to match the ultrasound.
What does 'term' mean in pregnancy?
A pregnancy is considered 'term' between 37 and 42 weeks. 'Full term' is 39–40 weeks — the optimal window for delivery. 'Early term' is 37–38 weeks, 'late term' is 41 weeks, and 'post term' is 42+ weeks, at which point induction may be recommended. Babies born before 37 weeks are considered preterm.
What are the three trimesters of pregnancy?
The first trimester covers weeks 1–12, during which the embryo develops all major organs. The second trimester is weeks 13–26, when the pregnancy becomes more visible and fetal movement is felt. The third trimester is weeks 27–40, when the baby gains most of its birth weight and prepares for delivery.
What are signs that labour is near?
Signs that labour may be approaching include the baby 'dropping' lower into the pelvis (engagement), increased Braxton Hicks contractions, loss of the mucus plug, nesting instinct, back pain, and finally regular contractions that increase in frequency and intensity. Water breaking (rupture of membranes) is also a clear sign to call your midwife or doctor.