Weeks of Pregnancy: Calculation and Trimester-by-Trimester Guide

Finding out you are expecting a baby is one of life's most exciting moments. From that moment on, one of the first questions every expectant mother asks is: how many weeks pregnant am I? Correctly counting the weeks of pregnancy is essential for monitoring fetal development, scheduling medical appointments, and preparing for delivery. In this comprehensive guide, we will see how to calculate the weeks, what happens trimester by trimester, and which appointments you should not miss.

How pregnancy weeks are counted

The counting of pregnancy weeks does not start from the moment of conception, as one might think, but from the first day of the last menstrual period (known as the LMP date). This method is the international medical standard because the date of conception is often uncertain, while the date of the last period is generally known precisely.

In practice, when the gynecologist says you are "6 weeks along," it means that 6 weeks have passed since the start of your last menstrual period. Actual conception usually occurs around the second week, so gestational age is always about 2 weeks more than the actual age of the embryo.

Pregnancy lasts an average of 40 weeks, or 280 days from the LMP date, equal to approximately 9 months and 7 days on the calendar. However, a birth is considered full-term if it occurs between the 37th and 42nd week.

Naegele's Rule

The most classic method for estimating the expected due date (EDD) is Naegele's Rule, formulated by the German obstetrician Franz Karl Naegele in the 19th century. The formula is simple:

Expected due date = Last menstrual period date + 7 days - 3 months + 1 year

Let's work through a practical example. If your last menstrual period started on January 15, 2026:

• January 15 + 7 days = January 22
• January 22 - 3 months = October 22
• The expected due date is October 22, 2026

This rule assumes a regular 28-day cycle. If your cycle is longer or shorter, the gynecologist will make adjustments. For an immediate and accurate calculation, you can use our online pregnancy calculator, which also takes these variables into account.

First trimester: weeks 1-12

The first trimester is the most delicate period of pregnancy, during which all the baby's major organs are formed.

Weeks 1-4: the invisible beginning

In the first two weeks, there is not yet a pregnancy in the strict sense: the body prepares for ovulation. Conception occurs around day 14. By the end of the fourth week, the embryo has implanted in the uterus and is the size of a poppy seed (about 1 mm). It is during this period that the pregnancy test becomes positive, detecting the beta-HCG hormone.

Weeks 5-8: the heart forms

The embryo grows rapidly. By the fifth week, the heart begins to beat, although it is still too small to be heard. By the end of the eighth week, it measures approximately 1.5-2 cm and weighs about 1 gram. The limbs, eyes, and ears begin to form. This is the period when many women experience morning sickness, fatigue, and breast tenderness.

Weeks 9-12: from embryo to fetus

From the ninth week onward, it is officially called a "fetus." By the end of the first trimester, the baby measures approximately 6-7 cm and weighs about 20 grams. All major organs are formed and beginning to function. The fingers and toes are separated and the fetus begins to move, although the mother cannot feel it yet.

Important appointments in the first trimester:

• First gynecological visit (week 6-8): pregnancy confirmation with ultrasound
• Combined test and nuchal translucency (weeks 11-13): screening for chromosomal abnormalities
• Complete blood tests: blood type, complete blood count, blood sugar, toxoplasmosis, rubella, HIV

Second trimester: weeks 13-27

The second trimester is often called the "golden period" of pregnancy. Nausea generally disappears, energy returns, and the baby bump begins to show.

Weeks 13-16: first movements

The fetus grows rapidly, going from about 7 cm to 12 cm. By the sixteenth week, it weighs approximately 100 grams. Fingerprints develop and the fetus begins to swallow amniotic fluid. Second-time mothers may begin to feel the first movements as early as week 16.

Weeks 17-20: finding out the sex

By the twentieth week, the fetus measures approximately 25 cm from head to toe and weighs about 300 grams. The anatomy scan, performed between weeks 19 and 21, is one of the most exciting appointments: it verifies the correct formation of all organs and, if the baby cooperates, reveals the sex.

Weeks 21-27: growth and sensory development

The baby begins to hear external sounds, recognizes the mother's voice, and reacts to light. By the end of the second trimester, it weighs approximately 900 grams to 1 kg and measures about 35 cm. The lungs begin to produce surfactant, a substance essential for breathing after birth.

Important appointments in the second trimester:

• Anatomy scan (weeks 19-21): detailed examination of all organs
• Glucose tolerance test (weeks 24-28): screening for gestational diabetes
• Urine test and follow-up blood count
• Possible amniocentesis (weeks 15-18) if indicated by screening

Third trimester: weeks 28-40

The last trimester is dedicated to the growth and maturation of the baby's organs, as it prepares for life outside the uterus.

Weeks 28-32: the baby turns

The fetus weighs approximately 1.5 kg at week 28 and reaches 2 kg by week 32. During this period, it usually positions itself head-down (cephalic presentation) in preparation for delivery. The brain develops rapidly and the baby alternates between regular sleep and wake phases.

Weeks 33-36: lung maturation

The lungs complete their maturation. The baby accumulates subcutaneous fat that will help regulate temperature after birth. By week 36, it weighs approximately 2.5-2.8 kg and measures about 47 cm.

Weeks 37-40: ready to be born

From week 37, the pregnancy is considered "full-term." The baby continues to grow, reaching an average of 3-3.5 kg and 50 cm at birth. The head engages in the pelvis and the mother may notice that the bump "drops."

Important appointments in the third trimester:

• Growth ultrasound (weeks 30-32): checking growth and position
• Vaginal-rectal swab (weeks 36-37): screening for Group B streptococcus
• Cardiotocographic monitoring (from full-term): monitoring fetal heartbeat and contractions
• Visits every 2 weeks, then weekly from full-term

Summary table of fetal development

How to use the pregnancy calculator

To know immediately and precisely how many weeks along you are and when the due date is expected, you can use our online pregnancy calculator. Simply enter the date of your last menstrual period and, if you know it, the average length of your cycle. The calculator will provide you with:

• The exact week of pregnancy you are in
• The expected due date
• The current trimester
• A summary of the key milestones already reached and those still to come

Practical tips for each trimester

First trimester

Start taking folic acid right away (400 mcg per day, or 5 mg if recommended by your doctor) to prevent neural tube defects. Avoid alcohol, smoking, and high-risk foods (raw meat, unpasteurized cheeses, raw fish). Rest is important: first-trimester fatigue is normal and physiological.

Second trimester

This is the ideal time for gentle exercise: walks, swimming, prenatal yoga. Diet should be varied and rich in iron, calcium, and protein. The recommended weight gain in the second trimester is approximately 400-500 grams per week.

Third trimester

Pack your hospital bag by week 36. Attend a childbirth preparation class to learn breathing and relaxation techniques. Sleep on your left side to promote circulation and discuss a birth plan with your gynecologist.

When the expected date does not match

It is important to know that only 5% of babies are born on the exact expected date. Most are born within 2 weeks before or after. If the first-trimester ultrasound indicates a different date from the one calculated with Naegele's Rule, the gynecologist may update the expected due date. The crown-rump length (CRL) measurement at the 11-13 week ultrasound is the most reliable parameter for dating a pregnancy.

Frequently Asked Questions

How many weeks does a pregnancy last exactly?

Pregnancy lasts an average of 40 weeks (280 days) calculated from the date of the last menstrual period. However, it is considered full-term between weeks 37 and 42, so a margin of 5 weeks is perfectly normal.

If I have an irregular cycle, how do I calculate the weeks?

With an irregular cycle, the most reliable dating method is the first-trimester ultrasound. The ultrasound between weeks 8 and 13 has a margin of error of only 3-5 days. Our pregnancy calculator also allows you to enter the average length of your cycle for a more accurate estimate.

Do pregnancy weeks and months correspond?

Not exactly. A calendar month averages 4.3 weeks, not 4. Here is the approximate correspondence: the first month goes from week 1 to week 4, the second from week 5 to week 8, the third from week 9 to week 13, and so on until the ninth month which covers weeks 36 to 40.

What is gestational age and what is conceptional age?

Gestational age is counted from the last menstrual period and is the one used in medical settings. Conceptional age is counted from conception and is approximately 2 weeks less. When the gynecologist indicates the weeks, they always refer to gestational age.

When should I have the first ultrasound?

The first ultrasound is generally performed between weeks 6 and 8. At this stage, intrauterine pregnancy is confirmed, the heartbeat is verified, and the pregnancy is dated. Before week 6, it may be too early to visualize the embryo.

What happens if I go past week 40?

If you pass the expected due date, the gynecologist intensifies monitoring with cardiotocographic checks and amniotic fluid assessment. Generally, if labor does not begin spontaneously by weeks 41-42, induction of labor is considered to ensure the safety of both mother and baby.