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Week Breakdown
| Week | From | To | Trimester |
|---|---|---|---|
| Week 1 | MMM D | MMM D, YYYY | T1 |
| Week 2 | MMM D | MMM D, YYYY | T1 |
| Week 3 | MMM D | MMM D, YYYY | T1 |
| Week 4 | MMM D | MMM D, YYYY | T1 |
| Week 5 | MMM D | MMM D, YYYY | T1 |
Understanding Your Pregnancy Week by Week
Pregnancy is measured in weeks because it provides a more precise way to track your baby's development than months. Our pregnancy week calculator helps you understand exactly where you are in your journey, from the first day of your last menstrual period through your due date at 40 weeks. Whether you're curious about your current progress or planning for upcoming milestones, knowing your exact week and day helps you follow along with your baby's amazing growth.
What's the Difference Between Gestational Age and Fetal Age?
When your doctor says you're 8 weeks pregnant, they're using gestational age—counting from the first day of your last period. But your baby has actually only been developing for about 6 weeks since conception typically occurs around day 14 of your cycle. This 2-week difference often confuses new parents. Fetal age (or embryonic age) counts from actual conception. Medical professionals use gestational age because it's easier to identify your LMP than your exact conception date. Our calculator shows both your gestational week and the corresponding developmental stage. To find when conception occurred, try our conception calculator.
Why Do Doctors Use "Weeks + Days" Format?
You'll often hear your pregnancy described as "12 weeks and 3 days" or written as "12+3." This precise notation matters because significant developments happen within single weeks, especially early in pregnancy. For example, a baby at 23+6 and one at 24+0 are only one day apart, but 24 weeks marks the threshold of viability—when survival outside the womb becomes possible with intensive care. Similarly, the window for certain tests like the nuchal translucency screening is between 11+0 and 13+6 weeks. This precision helps ensure proper timing for prenatal care and accurately assessing your baby's development.
How Do Trimesters Divide the 40 Weeks?
Pregnancy is divided into three trimesters, each with distinct characteristics and milestones. The First Trimester (weeks 1-12) covers the period of most rapid development when all major organs form. The Second Trimester (weeks 13-26) is often called the "honeymoon period" as many early symptoms ease and you'll likely feel your baby's first movements. The Third Trimester (weeks 27-40) is focused on growth and preparation for birth. Understanding which trimester you're in helps you know what to expect for symptoms, which tests are coming up, and what's happening with your baby's development. For a detailed look at when your baby will arrive, check our due date calculator.
Converting Weeks to Months: Why It's Confusing
"How many months pregnant are you?" seems like a simple question, but the answer is surprisingly complex. A lunar month is about 4 weeks, but calendar months vary from 28-31 days. This means 40 weeks of pregnancy equals roughly 9 calendar months and 1 week—not exactly 9 months. Different sources use different conversion methods, which is why your doctor sticks to weeks for precision. When people ask your month, a rough guide is: months 1-3 = first trimester, months 4-6 = second trimester, months 7-9+ = third trimester. For exact conversions, our pregnancy month calculator can help translate between weeks and months.
Frequently Asked Questions
When does pregnancy officially start?
Medically, pregnancy is dated from the first day of your last menstrual period (LMP), even though conception happens about 2 weeks later. This means during "week 1 and 2" of pregnancy, you're not actually pregnant yet—you're approaching ovulation. This dating convention exists because LMP is a known date, while conception date is usually estimated.
Why might my week count differ from the ultrasound?
Ultrasounds measure your baby's size to estimate gestational age. If you ovulated earlier or later than the assumed day 14, or if your baby is measuring slightly larger or smaller than average, the ultrasound-based age may differ from your LMP calculation. If there's more than a 5-7 day difference early on, your provider may adjust your official due date.
Is week 1 or week 0 the first week of pregnancy?
Week 1 is the first week. There is no "week 0" in pregnancy dating. When you're 0 days into week 1, you're at 1+0. When you complete week 1 and enter week 2, you're at 2+0. It's like age—a newborn is in their first year of life, not their zeroth.
How accurate is the pregnancy week calculation?
If you have regular cycles and know your LMP, the calculation is quite accurate—typically within a few days to a week. Irregular cycles, uncertain LMP dates, or early/late ovulation can affect accuracy. An early ultrasound (7-12 weeks) provides the most accurate dating, usually within 3-5 days.
Why does my app show a different week than my doctor?
Different apps may use slightly different calculation methods or rounding rules. Your doctor's dating is based on your official due date (which may have been adjusted by ultrasound), while apps typically calculate from the LMP you entered. Always go with your healthcare provider's assessment for medical decisions.
At what week is it safe to tell people I'm pregnant?
Many people wait until after 12 weeks (end of first trimester) when miscarriage risk drops significantly. However, this is a personal choice. Some share early for support, others wait until 20 weeks after the anatomy scan. There's no medical "requirement"—share whenever you feel comfortable.