Pregnancy Calculator
Track your pregnancy with accuracy and ease using our advanced Pregnancy Calculator. Discover your estimated due date, important trimester milestones, and weekly progress. Designed to guide and inform you through every stage of your journey to motherhood.
This is the first day of your most recent menstrual period, not when it ended.
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How It Works
How Pregnancy Is Calculated
Pregnancy calculation often surprises people because the first two weeks of pregnancy occur before conception. Medical professionals calculate pregnancy duration from the first day of your last menstrual period (LMP), not from the day of conception.
A typical pregnancy lasts approximately 40 weeks (280 days) from the LMP, or about 38 weeks (266 days) from conception. This method of dating pregnancy was established because most women know when their last period began, but the exact date of conception is often uncertain.
The Role of LMP and Conception Date
Last Menstrual Period (LMP): This is the standard starting point for calculating a pregnancy. Even though you're not actually pregnant during your period, these two weeks before conception are counted as part of your pregnancy.
Conception Date: This typically occurs about two weeks after the LMP, during ovulation. If you know your conception date (through fertility tracking or fertility treatments), your pregnancy is usually dated as conception date plus 266 days.
IVF Transfer Date: For pregnancies achieved through in vitro fertilization (IVF), the calculation is based on the age of the embryo at transfer and the transfer date. For a 3-day embryo, the due date is the transfer date plus 263 days. For a 5-day blastocyst, it's the transfer date plus 261 days.
How Trimesters Are Defined
Pregnancy is divided into three trimesters, each approximately 13 weeks long:
- First Trimester (Weeks 1-12): Begins on the first day of your LMP and lasts until the end of week 12. This is when all major organs and structures begin to form.
- Second Trimester (Weeks 13-27): Often called the "golden period" of pregnancy when many women feel their best. The baby grows significantly, and movements become noticeable.
- Third Trimester (Weeks 28-40+): The final stretch of pregnancy when the baby gains weight and prepares for birth. Full term is considered 39-40 weeks.
What to Expect in Each Trimester
First Trimester (Weeks 1-12)
The first trimester is characterized by significant development of the embryo and later, the fetus. For the mother, it often brings:
- Morning sickness (nausea and vomiting)
- Extreme fatigue
- Breast tenderness
- Frequent urination
- Food aversions or cravings
- Mood swings due to hormonal changes
By the end of the first trimester, the fetus is about 3 inches long and weighs approximately 1 ounce. All essential organs have begun to form.
Second Trimester (Weeks 13-27)
The second trimester is often considered the most comfortable. Developments include:
- Baby's movements become noticeable (quickening)
- Baby's sex may be detectable via ultrasound
- Mother's energy levels typically increase
- Morning sickness usually subsides
- Weight gain becomes more noticeable
- The "pregnancy glow" often appears
By the end of the second trimester, the fetus is about 14 inches long and weighs approximately 2 pounds. The baby develops hair, eyebrows, and eyelashes.
Third Trimester (Weeks 28-40+)
The final stretch of pregnancy brings:
- More pronounced fetal movements
- Braxton Hicks contractions
- Increased back and pelvic pain
- Shortness of breath as the baby grows
- Swelling in feet and ankles
- Difficulty sleeping
- Preparation for labor (lightening, nesting instinct)
By full term, the average baby is about 20 inches long and weighs approximately 7-8 pounds. The lungs and other vital organs are mature enough to function outside the womb.
Frequently Asked Questions
How accurate is the due date calculation?
Due dates are estimates, not guarantees. Only about 5% of women deliver on their exact due date. Most births occur between 38 and 42 weeks of pregnancy. Due dates calculated from a known LMP are accurate within about 5 days, while ultrasound dating in the first trimester can be accurate within 3-5 days.
What if I don't know my LMP date?
If you don't know your LMP, your healthcare provider will likely order an ultrasound to determine gestational age. Early ultrasounds (before 20 weeks) provide the most accurate dating of a pregnancy when LMP is unknown.
Why is fetal age different from gestational age?
Fetal age refers to the actual age of the baby from conception, which is approximately 2 weeks less than gestational age. Gestational age is measured from the LMP and is the medical standard for dating pregnancies, even though conception hasn't yet occurred during those first two weeks.
How is a due date determined with irregular periods?
For women with irregular cycles, LMP-based calculations may be less reliable. In these cases, healthcare providers typically use early ultrasound measurements to establish a more accurate due date.
What are Naegele's rule and how is it used?
Naegele's rule is the standard method for calculating a due date: Take the first day of the LMP, subtract 3 months, and add 7 days (plus one year if applicable). For example, if the LMP was August 10, 2024, the due date would be May 17, 2025. This calculation assumes a 28-day cycle with ovulation on day 14.
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