Pregnancy Calculator

Pregnancy Timeline by Trimester

Important Pregnancy Milestones

Understanding Pregnancy Calculations

Naegele's Rule

Definition: The standard method for calculating estimated due date (EDD). Add 280 days (40 weeks or 9 months + 7 days) to the first day of the last menstrual period (LMP).

History: Developed by German obstetrician Franz Naegele in 1812. Assumes a regular 28-day menstrual cycle with ovulation on day 14. Still the primary method used by healthcare providers worldwide.

Accuracy: Only about 5% of babies are born on their exact due date. Most are born within 2 weeks before or after. The "due date" is better understood as an estimated delivery month. First-time mothers often deliver after 40 weeks; subsequent pregnancies slightly earlier.

How Pregnancy Weeks Are Counted

From LMP, Not Conception: Pregnancy is counted from the first day of your last period, not conception date. This means you're considered 2 weeks pregnant at conception. Full-term pregnancy is 40 weeks from LMP, which equals 38 weeks from conception.

Why This Method: Most women know their last period date but not exact conception date. Ovulation timing varies (typically day 12-16 in 28-day cycle). Standardizing from LMP provides consistent medical dating across all pregnancies.

40 Weeks = 280 Days: Pregnancy lasts approximately 9 months and 7 days, or 10 lunar months (28-day months). In calendar months (30-31 days), it's closer to 9 months. This is why "9-month pregnancy" is common saying though medical standard is 40 weeks.

Trimesters Explained

First Trimester (Weeks 1-13): Highest risk of miscarriage (10-20% of known pregnancies). Embryo forms into fetus. Major organs and structures develop. Most women experience morning sickness, fatigue, and hormonal changes. Critical period for avoiding harmful substances.

Second Trimester (Weeks 14-27): Often called the "honeymoon period." Energy returns, morning sickness typically subsides. Baby bump becomes visible. Fetal movements felt (weeks 16-25, varies by pregnancy). Anatomy scan reveals detailed development. Risk of miscarriage much lower. Most women feel best during this trimester.

Third Trimester (Weeks 28-40+): Baby grows rapidly, gains most weight. Discomfort increases (back pain, frequent urination, difficulty sleeping). Braxton Hicks contractions may occur. Baby's lungs mature, preparing for breathing. Final preparations for labor and delivery. Weekly appointments near the end.

Conception Date Estimation

Calculation: Conception typically occurs about 2 weeks after LMP (around ovulation). If LMP was January 1, conception likely occurred around January 14. However, sperm can survive 5 days, and ovulation timing varies, so exact conception date is difficult to pinpoint.

Irregular Cycles: Women with irregular periods may need early ultrasound dating. First-trimester ultrasound (weeks 8-13) is most accurate for determining due date, accurate within 3-5 days. Later ultrasounds are less precise for dating.

Full Term Definitions

Early Term: 37-38 weeks 6 days. Baby is developed but benefits from additional time in womb. Higher risk of breathing issues and NICU admission compared to full term.

Full Term: 39-40 weeks 6 days. Optimal time for delivery. Lowest risk of complications. Baby's brain, lungs, and other organs have maximum time to develop. Most doctors prefer to wait until at least 39 weeks unless medical reason for earlier delivery.

Late Term: 41-41 weeks 6 days. Still considered safe. Many first-time mothers deliver in this range. Increased monitoring recommended.

Post-Term: 42 weeks or later. Increased risks (placenta aging, larger baby size, meconium aspiration). Induction typically recommended by 42 weeks. Occurs in about 5-10% of pregnancies.

Frequently Asked Questions

How accurate is the pregnancy calculator?

The calculator uses the standard Naegele's Rule method used by doctors worldwide. However, only 5% of babies are born on their exact due date. Most deliver within 2 weeks before or after (38-42 weeks). Due date is an estimate, not a guarantee. Early ultrasound (weeks 8-13) provides the most accurate dating, within 3-5 days.

Why is pregnancy 40 weeks but 9 months?

40 weeks equals 280 days, which is about 9 months and 7 days (9.3 months). Pregnancy is counted from first day of last period, not conception, adding about 2 weeks to actual fetal age. In 30-day calendar months, 40 weeks is closer to 9.3 months. The "9 months" is simplified rounding.

When did I conceive?

Conception typically occurs about 2 weeks after your last period started, around ovulation (day 14 of 28-day cycle). Sperm can survive up to 5 days, so conception window is several days around ovulation. Exact date is hard to pinpoint unless you tracked ovulation or had IVF with known fertilization date.

What if I don't know my last period date?

Your doctor will use early ultrasound to date the pregnancy. First-trimester ultrasound (weeks 8-13) is most accurate for determining due date. Crown-rump length measurement provides accurate estimate within 3-5 days. This becomes the official due date in your medical records.

Can my due date change?

Your doctor may adjust due date based on early ultrasound if it differs significantly from LMP-based date (more than 5-7 days). First-trimester ultrasound is considered more accurate than LMP for dating, especially with irregular cycles. Due date usually doesn't change after first-trimester ultrasound confirmation.

When will I feel baby move?

First-time mothers typically feel movement (quickening) around 18-25 weeks. Women who've been pregnant before often feel it earlier, around 16-18 weeks. Initially feels like flutters, bubbles, or gas. Becomes more distinct and regular as pregnancy progresses. Daily movement counts recommended in third trimester.

How long is each trimester?

First trimester: Weeks 1-13 (months 1-3). Second trimester: Weeks 14-27 (months 4-6). Third trimester: Weeks 28-40+ (months 7-9+). Each trimester is approximately 13-14 weeks or 3 months. Note: Some sources use slightly different week divisions, but this is most common.

What is considered full term?

Full term is 39-40 weeks 6 days. Early term is 37-38 weeks 6 days (baby can be born safely but benefits from more time). Late term is 41-41 weeks 6 days. Post-term is 42+ weeks (induction usually recommended). Babies born 37+ weeks are "term" but 39+ weeks is optimal.

Pregnancy Week by Week Overview

First Trimester Development

Weeks 1-4: Fertilization, implantation. Cells multiply rapidly. Pregnancy hormone (hCG) rises, allowing positive test. Weeks 5-8: Embryo develops heart, brain, spinal cord. Heartbeat begins (week 6). Limbs form. Morning sickness often starts. Weeks 9-13: Fetus now (no longer embryo). All organs present. Facial features form. Movement begins (not yet felt). Genitals develop.

Second Trimester Development

Weeks 14-17: Baby can make facial expressions. Skeleton hardens. Rapid growth. Weeks 18-22: Mother feels movements. Hearing develops. Vernix (protective coating) covers skin. Weeks 23-27: Baby responds to sound. Sleep/wake cycles establish. Lungs develop (still immature). Baby reaches about 2 lbs.

Third Trimester Development

Weeks 28-31: Eyes open, can see light. Brain developing rapidly. Weight gain accelerates. Weeks 32-36: Lungs mature. Baby positions for birth (usually head-down). Layer of fat forms. Weeks 37-40: Full term! All organs ready. Baby drops into pelvis. Ready for birth anytime. Average weight 7-8 lbs.

Important Pregnancy Tips

• Prenatal care: Schedule first appointment around 8 weeks. Regular checkups essential for monitoring baby's development and mother's health.
• Prenatal vitamins: Take daily, especially folic acid (400-800 mcg) to prevent neural tube defects. Start before conception if possible.
• Avoid harmful substances: No alcohol, smoking, or recreational drugs. Limit caffeine to 200mg daily (one 12oz coffee).
• Safe medications: Consult doctor before taking any medication, including over-the-counter. Many are unsafe during pregnancy.
• Healthy diet: Eat balanced meals with fruits, vegetables, whole grains, lean protein. Avoid raw/undercooked meat, fish, eggs. Limit fish high in mercury.
• Stay hydrated: Drink 8-10 glasses of water daily. Helps prevent constipation, urinary tract infections, and supports increased blood volume.
• Exercise: Moderate exercise (30 min daily walking, swimming, prenatal yoga) safe for most. Avoid contact sports and activities with fall risk.
• Weight gain: Recommended 25-35 lbs for normal BMI. 28-40 lbs for underweight. 15-25 lbs for overweight. 11-20 lbs for obese.
• Warning signs: Contact doctor for severe abdominal pain, vaginal bleeding, severe headache, vision changes, decreased fetal movement, or fluid leakage.

Common Pregnancy Symptoms by Trimester

First Trimester

Morning sickness (nausea/vomiting, any time of day). Extreme fatigue. Breast tenderness and swelling. Frequent urination. Food aversions and cravings. Mood swings. Mild cramping. Constipation.

Second Trimester

Energy returns, morning sickness usually subsides. Growing belly becomes obvious. Fetal movements felt. Back and hip pain. Leg cramps. Varicose veins may appear. Increased appetite. Skin changes (darkening, stretch marks).

Third Trimester

Shortness of breath (baby pushing on diaphragm). Frequent urination returns (baby on bladder). Difficulty sleeping. Back pain intensifies. Swelling in feet/ankles. Braxton Hicks contractions. Heartburn. Fatigue returns. Nesting instinct.