Pregnancy Week Calculator

How Pregnancy Weeks Are Counted

Pregnancy is measured from the first day of your last menstrual period (LMP), not from conception. This means during "weeks 1–2" you haven't actually conceived yet — ovulation happens around week 2, and conception occurs shortly after. Full-term pregnancy is 40 weeks from LMP.

What Happens Each Trimester

• First Trimester (Weeks 1–13): Most critical for organ formation. Morning sickness peaks at 8–10 weeks. Miscarriage risk highest here.
• Second Trimester (Weeks 14–27): The golden trimester — energy returns, bump shows, first movements (quickening) felt at 18–22 weeks.
• Third Trimester (Weeks 28–40): Rapid baby weight gain. Braxton Hicks, back pain, heartburn are common. Baby settles into birth position.

Why "Gestational Age" and "Fetal Age" Aren't the Same Number

This is one of the most common sources of pregnancy-week confusion: gestational age (what your doctor uses) is counted from your LMP, while fetal age (actual embryonic development) is counted from conception, roughly 2 weeks later. So when you're "6 weeks pregnant" by gestational age, the embryo itself is closer to 4 weeks of actual development. Both numbers are correct — they're just measuring from different starting points, and it's worth knowing which one any given source is using, especially when comparing week-by-week development guides.

How Your Weekly Experience Might Differ From "Typical"

Week-by-week guides describe averages, not universal rules. Some women feel first fetal movement (quickening) as early as 16 weeks, others not until 22-24 weeks, and both are entirely normal, influenced by factors like whether it's a first pregnancy and where the placenta is positioned. Similarly, symptoms like nausea, energy levels, and when the belly "shows" all vary widely between women and even between pregnancies for the same woman. Try to hold week-by-week content as a general guide rather than a strict checklist your own pregnancy needs to match.

Key Questions to Ask at Each Stage

• Early weeks (1-13): When should I schedule my first prenatal visit? Which foods and medications should I avoid?
• Mid pregnancy (14-27): What does my anatomy scan show? When should I feel movement, and what if I don't yet? What are my options for glucose screening?
• Late pregnancy (28-40): What are the signs of labor I should watch for? What's my plan for reaching my delivery location? What does my provider want me to know about Group B Strep testing?

What Happens in Each Trimester of Fetal Development

The first trimester (weeks 1-13) is when the foundational structures form — the neural tube, heart, and major organs all begin developing in these early weeks, which is exactly why a healthy diet and avoiding harmful exposures matter so much this early, often before many women even know they're pregnant. The second trimester (weeks 14-27) is typically when growth accelerates, movement becomes noticeable, and the anatomy scan confirms structural development in detail. The third trimester (weeks 28-40) is largely about weight gain, lung maturation, and the baby positioning for birth.

Viability Milestones Worth Understanding

Around 24 weeks is generally considered the threshold of viability outside the womb with intensive medical support, though outcomes improve significantly with each additional week. By 28 weeks, survival rates with NICU support are considerably higher, and by 34-37 weeks, most babies do very well with standard care if born early. These numbers can be reassuring context if you're ever facing a preterm labor scare, though every situation and gestational week carries its own specific considerations your care team will walk you through.

How This Calculator Handles Multiples

For twin or higher-order pregnancies, week-by-week development follows a similar biological timeline, but full term is defined earlier (around 37 weeks for twins rather than 40), since multiple pregnancies more commonly deliver ahead of a single-baby due date. If you're expecting multiples, your care team will likely set a more individualized monitoring schedule that adjusts standard week-by-week guidance to your specific situation.

About This Tool: Method, Limits & Sources

What it does. Shows how many weeks and days pregnant you are from the first day of your last period.

How it works. Days since the first day of the last period ÷ 7 gives completed weeks; the remainder is days. The trimester follows from the week (1st: up to 13 weeks; 2nd: 14–27; 3rd: 28 onward). Progress is shown as weeks out of 40.

Example. If your last period began 70 days ago, you are 10 weeks and 0 days pregnant (1st trimester, 25% of 40 weeks).

Limitations

• Gestational age counted from the last period assumes ovulation on day 14.
• Ultrasound dating can differ from the date calculated here.
• It does not show how the pregnancy is progressing.

When to Speak With a Healthcare Professional

• You have heavy bleeding, severe abdominal pain, fever, or decreased fetal movement — seek urgent care
• You want to confirm your due date — your provider usually does this with an ultrasound
• You have any concern about your pregnancy

References

• ACOG Committee Opinion 700 — Methods for Estimating the Due Date

Last updated: October 2026 · Published by Bloom Health Woman (not medically reviewed) · Editorial Policy