Did you know that pregnancy week by week reveals an extraordinary journey in which the fetus’s heart begins to beat as early as the sixth week? This is just one of the many miracles that take place during the 280 days (about 40 weeks) of this unique journey.
Pregnancy is divided into three distinct trimesters, each with specific characteristics and crucial developmental stages. Over the course of the pregnancy weeks, the embryo gradually transforms: by the ninth week it becomes a fetus measuring about 2.5 centimeters; by the twelfth week it reaches 7–8 centimeters and weighs around 25 grams; and by the thirteenth week it already measures 7–9 centimeters with a weight of 25–30 grams.
During pregnancy, we women experience numerous physical and emotional changes. It is common to experience nausea, fatigue, and increased sensitivity to smells as the body prepares to welcome a new life. Weight gain can range between 11 and 15 kilograms—a significant but necessary change.
In this complete guide to gestational weeks, we will explore together every stage of this extraordinary journey, from the earliest signs to the moment of birth. We will discover how to correctly calculate pregnancy weeks, which tests to undergo at the right times, and how to take care of our well-being during this special yet demanding period.
How Pregnancy Weeks Are Calculated
Accurately calculating pregnancy weeks is one of the first aspects that often sparks curiosity when discovering you are pregnant. Although many people ask, “What month are you in?”, medical monitoring of pregnancy is primarily based on weeks, ensuring greater precision in tracking the baby’s development.
Average Length of Pregnancy
Pregnancy has a standard duration of 40 weeks, equivalent to 280 days. This corresponds to the well-known 9 months—or more precisely, 10 lunar months of 28 days each. However, this is only a general guideline, as most babies are born before or after the estimated due date.
Births occurring between the 37th and 41st week are considered normal. If birth occurs before the 37th week, it is classified as preterm, while births after the 42nd week are considered post-term.
To calculate the estimated due date, doctors often use Naegele’s rule: add 7 days and subtract 3 months from the first day of the last menstrual period. For example, if the last menstrual period began on August 21, the estimated due date would be May 28.
Difference Between Gestational Age and Conception Age
In medical practice, two methods are used to calculate the baby’s age:
- Gestational age: calculated from the first day of the last menstrual period
- Conceptional (or embryonic) age: calculated from the day of conception
Gestational age is the standard method used by doctors, even though it includes about two weeks when pregnancy has not yet actually occurred. This is because conception typically happens 14–15 days after the start of the last menstrual period, during ovulation.
This method is used because the date of the last menstrual period is usually certain, whereas the exact date of conception often is not. Most women also discover they are pregnant around the fourth or fifth week—about one month after the last cycle.
During gynecological visits, gestational age is expressed using a specific notation, such as “10+4,” meaning 10 weeks and 4 days. For clarity, the tenth week runs from 9+0 to 9+6.
Using a Pregnancy Calculator
The obstetric wheel, also known as the pregnancy wheel, is a particularly useful tool for accurately calculating pregnancy weeks and the estimated due date. It consists of two overlapping wheels: the lower one shows the months and days of the year, while the upper one indicates pregnancy weeks.
To use it, simply align the start of the first week with the first day of the last menstrual period; the arrow at the 40th week will indicate the estimated due date. Gynecologists and midwives regularly use this tool to determine when to schedule tests and checkups.
In cases of irregular cycles or uncertainty about the date of the last menstrual period, a first-trimester ultrasound (ideally between the 9th and 16th week) becomes essential for determining gestational age more accurately.
The Three Trimesters: What to Expect in Each Phase
Pregnancy is a journey divided into three distinct phases, each characterized by significant changes for both mother and baby. Knowing what to expect in each trimester can help you navigate this journey with greater awareness and peace of mind.
First Trimester: Early Symptoms and Embryonic Development
The first trimester runs from week 0 (the first day of the last menstrual period) to week 13+1. During these early weeks, the embryo develops rapidly in an environment that is still invisible externally, while the mother’s body begins to show the first signs of pregnancy.
Common symptoms include:
- Absence of menstruation
- Nausea and vomiting, caused by the hormone beta-hCG
- Fatigue and drowsiness
- Breast tenderness and enlargement
- Mood swings due to hormonal fluctuations
- Implantation bleeding
During this time, the embryo develops from a single fertilized cell into a being with all major organs formed. By the sixth week, the heart is already beating about 110 times per minute. By the end of the trimester, the fetus measures about 6.5 centimeters and weighs around 73 grams.
The greatest risk during this period is miscarriage, which is more common in the first trimester. A healthy lifestyle is therefore essential: taking folic acid (0.4 mg daily), avoiding alcohol and smoking, eating a balanced diet, and attending regular checkups.
Second Trimester: Growth and First Movements
The second trimester (weeks 13+2 to 26+2) is often considered the “golden period” of pregnancy. Early discomforts tend to subside, energy returns, and hormonal levels stabilize.
The uterus expands, the belly becomes visible, and many women experience increased appetite, mild back pain, heartburn, and an overall sense of well-being.
One of the most exciting moments is feeling the baby’s first movements, usually between the 19th and 20th week. Initially perceived as gentle flutters, they gradually become stronger and more distinct.
During this phase, the fetus grows rapidly, the placenta is fully formed, and the baby’s sex can often be identified via ultrasound. By the end of the trimester, the baby weighs about 550 grams, and all sensory organs are fully developed.
Third Trimester: Preparing for Birth
The third trimester (weeks 26+3 to 40) focuses on preparing for childbirth. The mother’s body continues to adapt to accommodate the growing baby.
Common changes include shortness of breath, Braxton Hicks contractions, possible colostrum leakage, frequent urination, and swelling of the legs. Emotionally, this period can be marked by both excitement and anxiety as birth approaches.
By the end of this phase, the baby usually weighs about 3–3.5 kilograms and is positioned head-down in preparation for delivery.
Conclusion
By the end of this journey through pregnancy weeks, it becomes clear that each phase represents a unique and precious moment in the creation of new life. Over these nine months, the body undergoes extraordinary transformations while the baby develops from a single embryo into a fully formed human being.
Every pregnancy is a deeply personal and unique experience. While general guidelines exist, development timelines and symptoms can vary greatly from one woman to another. Regular medical checkups, balanced nutrition, moderate physical activity, and emotional support all play a crucial role in ensuring a healthy pregnancy.
Ultimately, pregnancy week by week teaches patience, resilience, and unconditional love. Taking the time to fully experience each moment allows us to embrace this extraordinary adventure—knowing that the real journey begins with the arrival of our baby.
