Last Menstrual Period
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When Is Your Baby Due?
One of the first questions every expecting parent asks: when will the baby arrive? Our pregnancy due date calculator estimates your expected delivery date (EDD) using the same method obstetricians use — Naegele's Rule — based on the first day of your last menstrual period (LMP) or your known conception date.
Enter your LMP date or conception date, and the calculator provides your estimated due date along with a week-by-week trimester breakdown, current gestational age, and key milestone dates.
How it works: From your LMP, the calculator adds 280 days (40 weeks) to estimate your due date. From a known conception date, it adds 266 days (38 weeks). Both methods produce the same result because conception typically occurs approximately 14 days after the start of your last period.
How Due Dates Are Calculated
Naegele's Rule is the standard method used worldwide. It works by taking the first day of your last menstrual period, adding one year, subtracting three months, and adding seven days. This equals 280 days or 40 weeks from your LMP.
The underlying assumption is a 28-day menstrual cycle with ovulation occurring on day 14. If your cycle is longer or shorter than 28 days, your actual ovulation — and therefore conception — may differ from the standard assumption, shifting your true due date by a few days.
IVF and known conception dates provide the most precise dating. If you know the exact date of conception (or embryo transfer), the calculation is simpler — add 266 days (38 weeks) from conception. For IVF pregnancies, your clinic will provide specific dating based on embryo age at transfer.
Ultrasound dating is the most accurate method in early pregnancy. A first-trimester ultrasound (before 13 weeks) measures the embryo's crown-rump length and can estimate gestational age within 3–5 days. If the ultrasound date differs from the LMP-based date by more than 7 days, your healthcare provider will typically adjust the due date to match the ultrasound.
It's important to understand that only about 5% of babies are born on their exact due date. The due date is the midpoint of a normal delivery window. Full-term delivery ranges from 37 weeks (early term) to 42 weeks (late term), with most babies arriving between 39 and 41 weeks.
Trimester Breakdown
First trimester (Weeks 1–12): This period begins from the first day of your last period, even though conception doesn't occur until around week 2–3. Major organ development happens during this trimester — the heart starts beating around week 6, and by week 12, all major organs and body systems are formed. This is when morning sickness is most common (typically weeks 6–14) and when the risk of miscarriage is highest (dropping significantly after week 12 with a confirmed heartbeat).
Second trimester (Weeks 13–26): Often called the "golden period" because morning sickness typically subsides and energy returns. The baby grows rapidly — you may feel first movements (quickening) between weeks 16–22. The anatomy scan at 18–22 weeks checks the baby's development and may reveal the sex. The baby reaches viability (able to survive outside the womb with intensive medical care) around 24 weeks, though outcomes are significantly better at later gestational ages.
Third trimester (Weeks 27–40): The baby gains weight rapidly, brain development accelerates, and lungs mature. Braxton Hicks contractions (practice contractions) become common. The baby typically settles into a head-down position by 36 weeks in preparation for delivery. From 37 weeks onward, the baby is considered early-term, and from 39 weeks, full-term.
Key Pregnancy Milestones by Week
Several important dates cluster around specific weeks. Week 8–10 is when many parents have their first prenatal appointment and may hear the heartbeat for the first time. Week 10–13 is when non-invasive prenatal testing (NIPT) can screen for chromosomal conditions. Week 12 marks the end of the first trimester and a significant drop in miscarriage risk.
Week 18–22 is the anatomy scan — a detailed ultrasound that checks the baby's development. This is often when parents learn the baby's sex if they choose to. Week 24 marks viability — the point at which a baby has a reasonable chance of survival if born early, though significant medical intervention would be needed.
Week 28 begins the third trimester. Week 36 is when your provider may begin weekly appointments and check the baby's position. Week 37 is early term. Week 39 is full-term, and this is the earliest that elective inductions and scheduled cesarean sections are typically performed (unless medically indicated earlier).
Factors That Affect Your Due Date Accuracy
Irregular menstrual cycles are the most common reason LMP-based due dates are inaccurate. If your cycles are 35 days instead of 28, you likely ovulated around day 21 rather than day 14, making your actual due date about a week later than Naegele's Rule predicts. Our calculator allows you to input your average cycle length for a more accurate estimate.
Late ovulation can occur even in women with regular cycles due to stress, travel, illness, or hormonal fluctuation. If ovulation was delayed, the LMP-based due date will be earlier than the true date.
First pregnancies tend to run slightly longer — first-time mothers deliver an average of 1–2 days past their due date compared to mothers who've given birth before.
Multiple pregnancies (twins, triplets) have shorter average gestational periods. Twins are typically delivered around 36–37 weeks, and triplets around 32–34 weeks. Due date calculations are the same, but expected delivery dates are adjusted earlier.
Maternal age, health conditions, and pregnancy complications can all influence when delivery occurs but don't change the due date calculation itself — they affect the likelihood of early or late delivery.
What Happens If You Go Past Your Due Date?
Going past your due date is common — approximately 30% of pregnancies extend beyond 40 weeks. Your healthcare provider will monitor you more closely with non-stress tests and amniotic fluid checks starting around 41 weeks.
Most providers recommend induction of labor between 41 and 42 weeks because the risk of complications increases after 42 weeks. These risks include decreased amniotic fluid, placental aging, increased baby size making delivery more difficult, and very rare but serious complications like stillbirth.
The decision about induction timing involves balancing these risks against the benefits of allowing labor to begin naturally. The ARRIVE trial (2018) showed that elective induction at 39 weeks in low-risk first-time mothers reduced cesarean rates without increasing adverse outcomes, leading some providers to offer elective induction at 39 weeks. Discuss your options and preferences with your healthcare provider.
Frequently Asked Questions
Due date calculators based on LMP are accurate to within about 2 weeks for most women with regular cycles. First-trimester ultrasound dating is more precise — accurate to within 3–5 days. Only about 5% of babies arrive on their exact due date. Most are born within a 2-week window on either side of the estimated date, which is perfectly normal.
Yes. If an early ultrasound shows the baby measuring significantly different from the LMP-based date (more than 7 days difference in the first trimester), your healthcare provider will typically adjust the due date to match the ultrasound. After the first trimester, due dates are rarely changed because baby size becomes more variable and less useful for dating.
Gestational age counts from the first day of your last menstrual period (LMP) — it includes approximately 2 weeks before conception actually occurred. Fetal age (embryonic age) counts from the date of conception. Gestational age is always about 2 weeks more than fetal age. When someone says "I'm 12 weeks pregnant," they mean 12 weeks gestational age, which is 10 weeks since conception.
Full-term pregnancy is 39 weeks 0 days through 40 weeks 6 days. Early term is 37 weeks 0 days through 38 weeks 6 days. Late term is 41 weeks 0 days through 41 weeks 6 days. Post-term is 42 weeks and beyond. Babies born at 39+ weeks generally have better outcomes than those born at 37–38 weeks, which is why most providers prefer to wait until 39 weeks for scheduled deliveries.
Add 266 days (38 weeks) to your conception date. If you conceived on January 15, your estimated due date is October 8. This is equivalent to the LMP method because LMP-based dating adds 280 days from a date approximately 14 days before conception. If you know your exact conception date, this method can be more accurate than the LMP method.
Yes. For a Day 3 embryo transfer, add 263 days to the transfer date. For a Day 5 blastocyst transfer, add 261 days. Your fertility clinic will provide the most accurate dating based on your specific protocol. IVF pregnancies have the most precise dating of all because the conception and transfer dates are known exactly.
Pregnancy is dated at 40 weeks from the first day of your last menstrual period, but actual fetal development spans approximately 38 weeks from conception. The extra 2 weeks account for the time between your period and ovulation. This convention dates back to the 19th century and remains the global standard for obstetric dating used by healthcare providers worldwide.
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