Week of Pregnancy – Meaning, Trimesters & Care

What Is the Week of Pregnancy?

The week of pregnancy, also referred to as gestational week or gestational age, is the standard unit used to measure the progress of a pregnancy. A normal pregnancy lasts approximately 40 weeks, counted from the first day of the woman's last menstrual period (LMP). This internationally accepted method is used by healthcare professionals worldwide to track fetal development and plan prenatal care.

Because the exact moment of conception is usually unknown, the count begins with the first day of the last menstrual period. Fertilisation and implantation typically occur around weeks 2 to 3 of the pregnancy.

Trimester Division

The 40 weeks of pregnancy are divided into three phases known as trimesters:

• First trimester (weeks 1 – 12): This is the embryonic phase, during which all major organs and body structures begin to form. The risk of miscarriage is highest during this period.
• Second trimester (weeks 13 – 27): The fetus grows rapidly, and the mother typically begins to feel fetal movements. Many women find this trimester more comfortable as early symptoms such as nausea often subside.
• Third trimester (weeks 28 – 40): A phase of continued growth and organ maturation. The fetus prepares for birth, and the lungs and other organs mature. The mother may experience physical discomfort such as back pain and difficulty sleeping.

Calculating the Week of Pregnancy

The current week of pregnancy is determined by the doctor or midwife using two main methods:

• Calculation from the last menstrual period: The weeks are counted forward from the first day of the last menstrual period.
• Ultrasound examination: An ultrasound (sonography) can precisely determine the gestational age of the fetus, especially in the first trimester. The crown-rump length (CRL) is the most important measurement used at this stage.

Importance for Prenatal Appointments

The week of pregnancy is closely linked to recommended prenatal check-ups. These examinations are scheduled at specific gestational weeks to monitor the health of both mother and child:

• Weeks 9 – 12: First ultrasound screening, blood tests, and confirmation of gestational age.
• Weeks 19 – 22: Detailed anatomy scan to assess the structure of fetal organs.
• Weeks 29 – 32: Routine check-up and fetal growth assessment.
• From week 36 onwards: Frequent monitoring in preparation for birth, including assessment of the baby's position.

Preterm Birth and Post-Term Pregnancy

A birth is classified as a preterm birth when it occurs before the completion of week 37. Premature infants may require intensive medical care depending on gestational age, as vital organ systems are not yet fully mature.

If birth has not occurred by the end of week 42, the pregnancy is considered post-term. In such cases, labour is typically induced medically to reduce risks for both mother and baby.

Clinical Relevance

Accurate knowledge of the gestational week is essential in obstetric practice. It guides the scheduling of prenatal examinations, assessment of fetal maturity, identification of growth disorders, and clinical decision-making in case of complications. The estimated due date (EDD) is standardly set at the completion of week 40.

References

1. World Health Organization (WHO): WHO recommendations on antenatal care for a positive pregnancy experience. Geneva: WHO Press, 2016.
2. American College of Obstetricians and Gynecologists (ACOG): Methods for Estimating the Due Date. Committee Opinion No. 700. Obstetrics & Gynecology, 2017.
3. Sadler TW: Langman's Medical Embryology. 14th edition. Philadelphia: Wolters Kluwer, 2019.