Finding out you’re pregnant comes with a rush of emotions — excitement, nervousness, joy, and inevitably, one big question: when is the baby due? Your estimated due date (EDD) becomes the anchor point for your entire pregnancy, determining when to schedule appointments, when to expect certain milestones, and when to start preparing for birth.

But due dates aren’t as precise as many people think. Here’s how they’re calculated, what they really mean, and how to track your pregnancy week by week.

How Due Dates Are Calculated: Naegele’s Rule

The standard method for calculating a due date is Naegele’s rule, named after the 19th-century German obstetrician Franz Naegele. The formula is based on a menstrual cycle of 28 days with ovulation occurring on day 14:

EDD = First Day of Last Menstrual Period (LMP) + 280 days (40 weeks)

For example, if your last period started on January 1:

January has 31 days, so add 31 → February 1
February has 28 days → March 1
Add 280 days total → October 8

An easier way to calculate: add 7 days to the LMP date, then subtract 3 months, then add 1 year.

LMP: January 1
Add 7 days: January 8
Subtract 3 months: October 8
Add 1 year: October 8

This method assumes ovulation on day 14 of a 28-day cycle. If your cycles are longer or shorter, the calculation needs adjustment:

Cycles longer than 28 days: Add the number of days beyond 28. For a 35-day cycle, add 7 days to the EDD.
Cycles shorter than 28 days: Subtract the number of days below 28. For a 24-day cycle, subtract 4 days from the EDD.

Alternative Methods for Dating a Pregnancy

Naegele’s rule works well when you have regular periods and know your LMP exactly. When that’s not the case, other methods can provide a more accurate estimate.

Conception Date

If you know the exact date of conception (common with IVF or fertility tracking), the due date is calculated by adding 266 days (38 weeks). This is more precise than LMP dating because it doesn’t rely on assumptions about cycle length.

Ultrasound Dating

An early ultrasound (particularly between 8–12 weeks) is considered the most accurate method for dating a pregnancy. The technician measures the crown-rump length (CRL) of the fetus and compares it to established growth charts.

Ultrasound dating becomes less accurate as pregnancy progresses because fetal growth rates begin to vary more. A first-trimester ultrasound is typically accurate to within 5–7 days, while a second-trimester measurement may be off by 10–14 days.

Most providers use the earliest reliable dating method. If your LMP and ultrasound dates differ by more than 5–7 days in the first trimester, the ultrasound date is generally preferred.

IVF Dating

For IVF pregnancies, the due date is calculated based on the embryo transfer date:

Day 3 transfer: EDD = Transfer date + 263 days
Day 5 transfer: EDD = Transfer date + 261 days

Why Only 5% of Babies Arrive on Their Due Date

It’s one of the most surprising pregnancy statistics: only about 5% of babies are born on their actual due date. The due date is an estimate, not a deadline.

Here’s when babies typically arrive:

Timing	Percentage of Births
Before 37 weeks (preterm)	~10%
37–38 weeks (early term)	~25%
39–40 weeks (full term)	~55%
41 weeks (late term)	~8%
42+ weeks (post term)	~2%

The average first-time mother delivers at about 41 weeks and 1 day. Subsequent births tend to come slightly earlier, around 40 weeks and 3 days.

This is why most providers don’t recommend induction before 41 weeks unless there’s a medical reason. Your body and your baby have their own timeline.

The Three Trimesters Explained

Pregnancy is divided into three trimesters, each with distinct developmental milestones and physical changes.

First Trimester (Weeks 1–12)

The foundation for everything. The fertilized egg implants in the uterus, the placenta forms, and all major organs begin to develop. By week 8, the embryo is officially a fetus with a beating heart, limbs, and facial features. This is also when morning sickness, fatigue, and breast tenderness are most common.

Key considerations: Avoid alcohol, smoking, and certain medications. Start taking prenatal vitamins with at least 400 mcg of folic acid. Schedule your first prenatal visit around week 8.

Second Trimester (Weeks 13–27)

Often called the “honeymoon phase” of pregnancy. Morning sickness usually fades, energy returns, and you start to feel the baby move (typically between weeks 16–22 for first pregnancies). The fetus grows rapidly, developing fingerprints, hair, and the ability to hear sounds.

Key considerations: This is when most genetic screening and anatomy scans happen (around weeks 18–22). Start thinking about childbirth classes and creating a birth plan.

Third Trimester (Weeks 28–40)

The home stretch. The baby gains most of its weight during this period, and the brain and lungs undergo critical development. You may experience back pain, swelling, Braxton Hicks contractions, and difficulty sleeping as the baby takes up more space.

Key considerations: Count kicks daily to monitor fetal activity. Pack your hospital bag around week 35. Know the signs of preterm labor and when to call your provider.

Key Pregnancy Milestones Timeline

Week	Milestone
4	Implantation complete; positive pregnancy test possible
8	Heartbeat detectable by ultrasound; all major organs forming
12	End of first trimester; risk of miscarriage drops significantly
16	Gender may be visible on ultrasound; baby is about 11 cm long
20	Anatomy scan; baby is about 25 cm long; you may feel movement
24	Viability milestone; baby has a chance of survival if born prematurely
28	Third trimester begins; baby’s eyes open; lungs continue developing
32	Baby practices breathing movements; bones are hardening
36	Baby is considered early term; head may engage in the pelvis
40	Full term; baby is typically 50 cm long and 3.2–3.6 kg

How to Prepare for Each Trimester

First trimester: Focus on nutrition and avoiding harmful substances. Choose a healthcare provider. Start a pregnancy journal to track symptoms and appointments.

Second trimester: Begin nursery planning. Research childcare options if needed. Start a registry. Take childbirth and breastfeeding classes. Exercise regularly with your provider’s approval.

Third trimester: Finalize your birth plan. Pre-register at the hospital. Prepare meals in advance for the postpartum period. Install the car seat. Wash baby clothes and organize essentials.

Common Questions About Due Dates

Can my due date change? Yes. If an early ultrasound suggests a different date than your LMP calculation, your provider may adjust your due date. This is most common in the first trimester.

What if I go past my due date? Most providers monitor closely after 40 weeks with non-stress tests and ultrasound checks. Induction is typically discussed at 41 weeks, though policies vary.

Are second babies born earlier? On average, yes — but only by a few days. Every pregnancy is different, so don’t count on it.

Is it safe to go past 40 weeks? Most pregnancies between 40–41 weeks are perfectly healthy. Risks increase gradually after 41 weeks, which is why your provider will monitor more closely.

Calculate Your Due Date

Use our free Pregnancy Calculator to estimate your due date based on your last menstrual period, see your pregnancy timeline, and track key milestones week by week.