Pregnancy Week Calculator
Enter the first day of your last menstrual period to calculate your current pregnancy week, trimester, estimated due date, and track your baby's developmental milestones.
Weeks 13–26. Baby grows rapidly. You may feel first movements.
Halfway point! Anatomy scan typically done this week.
Understanding Pregnancy Weeks: A Complete Guide
Tracking your pregnancy by week is one of the most common ways to monitor your baby's growth and development. From the moment of conception to delivery, each week brings new changes and milestones. Understanding where you are in your pregnancy helps you prepare for what's ahead and know what to expect at each stage.
How Pregnancy Weeks Are Counted
Pregnancy dating can be confusing because it begins from the first day of your last menstrual period (LMP), not from the date of conception. This means that during the first two weeks of 'pregnancy,' you aren't actually pregnant yet — ovulation and conception typically occur around week 2. This system is used because the LMP date is usually known with certainty, whereas the exact date of conception often isn't.
A full-term pregnancy lasts 40 weeks (280 days) from the LMP, which is approximately 38 weeks from the actual date of conception. Babies born between 39 and 40 weeks are considered 'full term,' while those born between 37 and 38 weeks are 'early term.' Births before 37 weeks are classified as preterm.
First Trimester (Weeks 1–12)
The first trimester is the most critical period of development. During these early weeks, the embryo's major organs and body systems begin to form. By week 4, the heart starts beating. By week 8, all major organs have begun developing, and the embryo is now called a fetus. By the end of the first trimester, the baby is about 3 inches long and weighs nearly an ounce.
This trimester is when many pregnancy symptoms are most intense, including morning sickness, fatigue, and breast tenderness. It's also the period with the highest risk of miscarriage, which is why many people wait until after week 12 to announce their pregnancy.
Second Trimester (Weeks 13–26)
Often called the 'golden trimester,' the second trimester is when many uncomfortable first-trimester symptoms subside. The baby grows rapidly during this period, developing fingerprints, facial expressions, and the ability to hear. Most parents feel their baby's first movements — called 'quickening' — between weeks 16 and 25.
The anatomy scan, typically performed around week 20, is one of the most anticipated ultrasounds. It checks the baby's organs, measures growth, and can often reveal the baby's sex. By the end of the second trimester, the baby is about 14 inches long and weighs approximately 2 pounds.
Third Trimester (Weeks 27–40)
The third trimester is the final stretch. The baby's brain develops rapidly, body fat accumulates to help regulate temperature after birth, and the lungs mature for breathing air. The baby's movements may feel different as space in the uterus becomes tighter — kicks and rolls replace the fluttery movements of earlier weeks.
During this trimester, prenatal visits become more frequent, typically every two weeks and then weekly in the final month. The baby usually moves into a head-down position in preparation for birth. By 40 weeks, the average baby weighs 7 to 8 pounds and is about 20 inches long.
Due Date Accuracy
The estimated due date is exactly that — an estimate. Only about 5% of babies are born on their actual due date. Most healthy pregnancies result in birth between 38 and 42 weeks. First-time mothers tend to deliver a few days after their due date on average. Factors like the accuracy of the LMP date, cycle length variations, and individual biological differences all affect when labor naturally begins.
If there's uncertainty about the LMP date, an early ultrasound (ideally before 14 weeks) provides the most accurate dating. This ultrasound measures the embryo's crown-rump length, which correlates closely with gestational age during early pregnancy.
Frequently Asked Questions
How are pregnancy weeks calculated?
Pregnancy weeks are counted from the first day of your last menstrual period (LMP), not from the date of conception. This means you are technically 'pregnant' for about 2 weeks before conception actually occurs. A full-term pregnancy is 40 weeks (280 days) from the LMP.
What's the difference between gestational age and fetal age?
Gestational age counts from the first day of the last menstrual period and is the standard used by healthcare providers. Fetal age (or conceptional age) counts from the actual date of fertilization, which is approximately 2 weeks less than the gestational age. When someone says they are '12 weeks pregnant,' they are referring to gestational age.
When is each trimester?
The first trimester spans weeks 1 through 12, the second trimester covers weeks 13 through 26, and the third trimester runs from week 27 until delivery (typically around week 40). Some sources define the trimesters slightly differently, but these are the most commonly used boundaries.
How accurate is the estimated due date?
Only about 5% of babies are born on their exact due date. Most births occur within two weeks before or after the due date. An early ultrasound (before 14 weeks) can refine the due date to within about 5-7 days of accuracy. The due date should be considered a general timeframe rather than a precise prediction.
What if my cycle is longer or shorter than 28 days?
The standard calculation assumes a 28-day menstrual cycle with ovulation on day 14. If your cycle is consistently longer or shorter, your actual ovulation date may differ, which can shift your true gestational age by a few days. Discuss your cycle length with your healthcare provider, who may adjust your due date based on early ultrasound measurements.