Pregnancy Calculator

Pregnancy calculation is a method that helps determine how many weeks pregnant you are and your estimated due date based on the date of your last menstrual period or ultrasound results. For expectant mothers, accurate pregnancy calculation is very important for monitoring the baby’s development and planning necessary tests and check-ups on time.

What Is a Pregnancy Calculator?

A pregnancy calculator is a digital tool that allows expectant mothers to enter their last menstrual period date, menstrual cycle length, in some cases intercourse date information, or available ultrasound measurements to learn their pregnancy week and estimated due date. This system takes the start date of pregnancy as a reference, enabling clear weekly tracking and helping mothers manage their pregnancy process in a more informed and controlled way.

Knowing the pregnancy week is essential for understanding the stage of the baby’s organ development, scheduling required screenings and tests at the correct time, and following regular doctor appointments.

How Is Pregnancy Calculated?

Pregnancy calculation is generally based on the first day of the last menstrual period (LMP). With this method, pregnancy duration is accepted as an average of 40 weeks (280 days). The estimated due date can be calculated by adding 7 days to the last menstrual date and subtracting 3 months. This method provides reliable results for women with regular cycles.

However, in cases of irregular cycles or when pregnancy is detected later, ultrasound measurements (especially first-trimester CRL measurements) provide more accurate results. Pregnancy week is extremely important for monitoring fetal development, planning medical tests, and ensuring timely medical follow-ups.

What Methods Are Used for Pregnancy Calculation?

More than one method can be used to calculate pregnancy, and the most accurate approach is determined according to the individual’s menstrual regularity and medical history. Data such as menstrual cycle, ovulation time, fertilization date, and ultrasound measurements may all be used in pregnancy calculation.

Although the most common method is calculation based on the last menstrual period, ultrasound data or ovulation information may provide more precise results in certain cases. The main methods used in pregnancy calculation are:

• Last menstrual period (LMP): This is the most commonly used method. The first day of the last menstrual bleeding is taken as the starting point, and approximately 40 weeks (280 days) are added to determine the estimated due date. For example, if your last menstrual period started on January 1st, your estimated delivery date would fall in early October.
• Ovulation or fertilization date: This method is preferred when the ovulation date or fertilization date is known. Approximately 38 weeks (266 days) are calculated from the fertilization date. In assisted reproductive treatments such as IVF, this method provides more precise results because the embryo transfer date is known.
• Ultrasound measurements: These provide highly reliable data, especially during the first trimester. The baby’s crown-rump length (CRL) is measured to determine gestational age. These findings are compared with calculations based on the last menstrual period to confirm pregnancy age and evaluate possible discrepancies.

How Many Weeks Pregnant Am I?

Pregnancy is medically evaluated in weeks, and fetal development, medical check-ups, and tests are planned accordingly. Therefore, knowing your exact pregnancy week is essential for healthy pregnancy monitoring.

Pregnancy week is usually calculated based on the first day of the last menstrual period. However, pregnancy week and pregnancy month are not exactly the same, which may cause confusion for expectant mothers.

Although pregnancy is commonly expressed in months in daily life, medical follow-up is based on weeks. For example, an 8-week pregnancy corresponds to approximately 2 months, while a 20-week pregnancy equals about 5 months. The week-to-month equivalences are as follows:

What Tests Are Performed by Weeks of Pregnancy?

Tests performed during pregnancy follow-up are very important for evaluating the baby’s development, detecting possible risks early, and planning a healthy pregnancy process. These tests are carried out for different purposes in each trimester:

Weeks 0–13: First Trimester Tests

The first trimester is a critical period when the baby’s basic organ development begins and the overall health of the pregnancy is evaluated. The main tests performed during this period include:

• Non-Invasive Prenatal Test (NIPT): An advanced screening test used to assess the risk of certain chromosomal abnormalities in the baby. It is usually performed from the 10th week of pregnancy onward.
• Double screening test (Weeks 12–13): Used to calculate the risk of chromosomal abnormalities such as Down syndrome. It is evaluated together with nuchal translucency measurement.

Weeks 14–27: Second Trimester Tests

The second trimester is the period when the baby’s anatomical development is examined in detail and certain structural risks are assessed. The main tests performed during this period include:

• Triple or quadruple screening test: Used to assess the risk of chromosomal abnormalities such as Down syndrome and Edwards syndrome, as well as neural tube defects.
• Detailed ultrasound (anomaly scan) (Weeks 18–23): The baby’s brain, heart, spine, and internal organ development are examined in detail.
• Glucose tolerance test (Weeks 24–26): Screening for gestational diabetes is performed.

Weeks 28–40: Third Trimester Tests

The last trimester covers the preparation for birth. Tests performed during this period aim to evaluate the baby’s well-being in the womb and readiness for delivery. The main tests include:

• NST (Non-stress test): The baby’s heart rate and movements are monitored to assess fetal well-being.
• Ultrasonography: The baby’s estimated birth weight, amniotic fluid volume, and placental condition are checked.
• Blood tests: Evaluation is performed for anemia, infections, and conditions that may affect delivery.
• Urinary tract infection screening: Urinalysis is used to monitor the presence of infection.
• Weight and blood pressure monitoring: The mother’s weight gain and blood pressure are regularly tracked. This helps detect conditions such as preeclampsia at an early stage.
• Rh incompatibility injection (Anti-D Immunoglobulin): For Rh-negative mothers, it is usually administered around the 28th week when necessary to prevent immune sensitization against the baby’s blood cells.

Frequently Asked Questions About Pregnancy Calculation

Why Is Pregnancy Calculated from the Last Menstrual Period?

Because the exact date of fertilization is often unknown, while the last menstrual period is easy to recall and provides a standardized starting point.

How Many Days Before a Period Is Pregnancy Unlikely?

Generally, the chance of becoming pregnant 3–4 days before a period is very low, but it is not completely impossible.

How Many Days After a Period Can You Get Pregnant?

On average, the likelihood of pregnancy is highest during ovulation, which occurs about 10–14 days after the start of the menstrual period.

Why Is Pregnancy Calculated as 40 Weeks?

Pregnancy duration is medically accepted as approximately 280 days (40 weeks) starting from the last menstrual period.

What Happens If Labor Does Not Start at 40 Weeks?

Under medical supervision, the mother and baby are closely monitored, and induction of labor may be planned if necessary.

At the Latest, Which Week Does Birth Occur?

Delivery can usually be awaited until the 41st week; pregnancies approaching the 42nd week are medically evaluated.