Due Date Calculator
Estimating Your Baby's Due Date
A due date isn't a deadline — it's an estimate at the center of a natural range. But knowing it gives you a framework for everything that follows: prenatal appointments, trimester milestones, birth plans, and the practical preparations that make the months ahead feel manageable rather than overwhelming.
How to Use This Calculator
Enter the first day of your last menstrual period (LMP) and your average cycle length. The standard assumption is a 28-day cycle with ovulation on day 14 — if your cycle is longer or shorter, the calculator adjusts the due date accordingly. Hit Calculate Due Date to see your estimated due date, current gestational age, trimester, and key milestone dates.
How Due Dates Are Calculated — Naegele's Rule
The standard method used by healthcare providers worldwide is Naegele's Rule:
For cycles other than 28 days, the adjustment is:
A 35-day cycle shifts the due date 7 days later; a 21-day cycle shifts it 7 days earlier. The formula is based on the assumption that conception occurs approximately 14 days after the LMP, though this varies.
The Three Trimesters
- First trimester (weeks 1–13) — the most critical developmental period. Major organs and structures form. Morning sickness and fatigue are most common here. Risk of miscarriage is highest in these weeks.
- Second trimester (weeks 14–27) — often called the "honeymoon trimester." Energy typically returns, morning sickness eases, and the pregnancy becomes visible. Fetal movement (quickening) is usually felt around weeks 18–22.
- Third trimester (weeks 28–40) — rapid weight gain for both baby and mother. Baby's lungs, brain, and immune system continue maturing. Braxton Hicks contractions begin. The focus shifts to birth preparation.
How Accurate Is the Due Date?
Only about 5% of babies are born on their exact estimated due date. The majority arrive within a window of 2 weeks before to 2 weeks after — a normal full-term pregnancy spans weeks 37–42. Births before 37 weeks are considered preterm; after 42 weeks is post-term.
The most accurate dating method is a first-trimester ultrasound (before 14 weeks), which measures the size of the embryo directly. If your ultrasound date differs significantly from the LMP calculation, your provider will likely use the ultrasound date as the official due date.
Key Pregnancy Milestones
- Week 6–8: First heartbeat detectable via ultrasound
- Week 10–12: First trimester screening (nuchal translucency scan, blood tests)
- Week 18–20: Anatomy scan ultrasound — gender can typically be seen
- Week 24: Viability threshold — survival outside the womb becomes possible with intensive medical support
- Week 28: Third trimester begins; Rhesus factor injection if applicable
- Week 36: Weekly prenatal visits typically begin
- Week 37: Full term — baby is considered ready for birth
- Week 40: Estimated due date
- Week 41–42: Post-term; induction discussions typically begin
Frequently Asked Questions
What if I don't know my last menstrual period date?
Your healthcare provider can estimate gestational age using an early ultrasound, which measures crown-rump length in the first trimester. This is actually more accurate than LMP dating in many cases, particularly if your cycles are irregular. If you have a recent positive pregnancy test, the approximate conception date can also provide a starting estimate.
Can I choose my due date or delivery date?
Elective inductions before 39 weeks are generally discouraged unless medically indicated. After 39 weeks, many providers will discuss elective induction, though policies vary. Scheduled C-sections are typically planned no earlier than 39 weeks for a healthy pregnancy. The safest deliveries happen when labor begins naturally.
My due date changed after my ultrasound — which is correct?
The ultrasound date is more reliable, especially if done in the first trimester. First-trimester ultrasounds are accurate to within 5–7 days. Second-trimester ultrasounds are accurate to within 2 weeks. If the difference between LMP dating and ultrasound dating exceeds these ranges, your provider will typically revise the due date to match the ultrasound.
What is gestational age vs. fetal age?
Gestational age is measured from the first day of your last menstrual period — so at the time of conception, you're already considered about 2 weeks pregnant. Fetal age (or embryonic age) counts from actual conception and is roughly 2 weeks less than gestational age. Healthcare providers use gestational age as the standard because the LMP is a known date while conception timing often isn't.
Is this calculator a substitute for medical advice?
No. This calculator provides an estimate based on standard obstetric formulas. Your due date should be confirmed by your healthcare provider, and any concerns about your pregnancy should be discussed with a qualified medical professional. Every pregnancy is different.