How Pregnancy Due Dates Are Calculated
Your estimated due date (EDD) is calculated using Naegele's Rule — a formula established in the 19th century that adds 280 days (40 weeks) to the first day of your last menstrual period (LMP). Despite its age, it remains the standard method used globally by obstetricians and midwives. The 280-day figure assumes a 28-day menstrual cycle with ovulation on day 14. Women with longer or shorter cycles may have due dates that differ slightly from this formula.
Formula: EDD = LMP + 280 days
Important note: Only about 5% of babies are born on their exact due date. Normal delivery occurs between 37 and 42 weeks. Babies born between 39–41 weeks are considered "full term." The due date is best understood as the midpoint of a probable 4–5 week delivery window.
Pregnancy by Trimester — What Happens When
| Trimester | Weeks | Key Developments |
|---|---|---|
| First | 1–13 | Organ formation begins, heartbeat detectable (~week 6), highest miscarriage risk period, morning sickness common |
| Second | 14–27 | Movement felt (~week 18–22), anatomy scan (~week 20), gender detectable, most report improved energy |
| Third | 28–40+ | Rapid weight gain, lung maturation, position settling, Braxton Hicks contractions, final preparation for birth |
Week-by-Week Pregnancy Milestones
- Week 4–5: Positive pregnancy test, embryo implants in uterine wall
- Week 6: Heartbeat detectable on ultrasound, embryo ~6mm
- Week 8: All major organs forming, embryo becomes a fetus
- Week 10: Fingernails form, can move limbs
- Week 12: End of highest-risk period, often when families share news
- Week 16: Fetus ~12cm, can make facial expressions
- Week 20: Anatomy scan (detailed ultrasound), approximately halfway point
- Week 24: Viability threshold — survival possible with intensive care
- Week 28: Third trimester begins, eyes open
- Week 32: Rapid brain development, practicing breathing movements
- Week 37: Early term — lungs nearly mature
- Week 39–40: Full term, optimal delivery window
Prenatal Appointments — Typical Schedule
Standard prenatal care in the US typically follows this schedule for a low-risk pregnancy:
- Weeks 4–8: First appointment — confirm pregnancy, establish care
- Every 4 weeks through week 28
- Every 2 weeks from weeks 28–36
- Every week from week 36 until delivery
High-risk pregnancies (multiple gestation, gestational diabetes, hypertension, advanced maternal age) typically require more frequent monitoring.
Due Date Accuracy and Alternative Calculation Methods
Ultrasound dating: A first-trimester ultrasound (before 13 weeks) can determine gestational age within ±5 days by measuring crown-rump length. This is considered more accurate than LMP-based calculation for women with irregular cycles. Second-trimester ultrasounds are less precise (±2 weeks).
Conception date: If you know your conception date, add 266 days (38 weeks). Conception typically occurs 10–14 days after the LMP in a 28-day cycle.
IVF dating: For IVF pregnancies, the transfer date is known precisely. Add 266 days from egg retrieval date or 268 days from the first day of the stimulation cycle.
Frequently Asked Questions
How accurate is the due date from LMP?
It is accurate within about ±2 weeks for most women with regular 28-day cycles. Women with irregular cycles or those who are unsure of their LMP may get a more accurate EDD from a first-trimester ultrasound. Most providers confirm or adjust the LMP-based EDD at the first scan.
What does it mean to be "full term"?
Full term is 39 weeks 0 days through 40 weeks 6 days. Early term is 37–38 weeks; late term is 41 weeks; post-term is 42 weeks or beyond. Outcomes are best for babies born in the full-term window. Elective deliveries before 39 weeks are generally not recommended unless medically indicated.
Can my due date change after the first ultrasound?
Yes. If the ultrasound measurement indicates a size significantly different from what the LMP-based EDD suggests (usually more than 5–7 days off), many providers will adjust the due date to match the ultrasound finding. Later ultrasounds are less accurate for dating and typically do not change an established due date.
What is gestational age vs fetal age?
Gestational age is counted from the LMP — so at the time of conception (approximately week 2–3), gestational age is already 2 weeks. Fetal age (or embryonic age) counts from conception and is typically 2 weeks less than gestational age. Medical providers always use gestational age.
What happens if I go past my due date?
Many healthy pregnancies extend past 40 weeks — this is normal. Most providers begin closer monitoring after 40 weeks and discuss induction options. The risk of complications rises modestly after 41–42 weeks, which is why most providers recommend induction by 42 weeks. Post-term pregnancy (42+ weeks) occurs in about 3–12% of pregnancies.
Can I calculate a due date for twins?
The due date calculation method is the same, but twin pregnancies have different expected delivery windows. Dichorionic-diamniotic (DCDA) twins are typically delivered at 38 weeks; monochorionic twins at 36–37 weeks due to shared circulation risks. This calculator gives the 40-week singleton EDD regardless of multiples — consult your provider for the appropriate target week for your specific twin pregnancy type.