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Pregnancy: the three trimesters

If you are pregnant: congratulations! Pregnancy is an incredible journey that comes with many physiological and psychological milestones. It may not always be easy, but certainly well worth it once the newborn arrives. The pregnancy period, measured from last menstrual period (LMP), takes approximately 40 weeks and is divided into three distinct stages known as trimesters. Each trimester represents a critical phase of development for the growing baby and brings physical and hormonal changes for the mother. Understanding the stages of pregnancy hopefully helps expectant parents prepare for what's ahead and recognize important physiological and psychological milestones along the way. We'd like to stress that each pregnancy is unique. There even have been cases of women only knowing they were pregnant once the baby was born! So if any of the what is discussed does not resonate with you, that's natural. If you have any concerns whatsoever, it is always advised to contact an obstetrician.

First trimester (weeks 1-12): early development

Most women will realize that they're pregnant around week 5, when the menstruation does not set in. This is the time to take a pregnancy test. This stage is crucial because the baby's major organs and nervous system begin to form. By the end of the first trimester:

• The heart has formed and is beating
• Arms, legs, toes and and fingers are present
• Vital organs like the liver, kidneys and lungs begin functioning

Common symptoms for the mother during this stage:

• Nausea and vomiting (morning sickness)
• Fatigue, increased need for sleep
• Frequent urination
• Mood swings
• Sensitive breasts

Second trimester (weeks 13-26): middle stage

Sometimes referred to as the "golden period" of pregnancy, the second trimester is when many women start to feel better physically. Energy levels typically improve, and early pregnancy symptoms may subside. The baby is growing and starts to develop more detailed characteristics:

• Gender detection is possible via ultrasound around week 20
• Baby can sometimes be felt moving
• Hearing develops and the baby can respond to sounds from outside the womb.
• Bones harden and muscles strengthen
• Hair and fingerprints form

Common symptoms for the mother during this stage:

• Reduced fatigue and nausea
• Growing belly and visible pregnancy
• Increased appetite
• Higher libido
• Skin changes such as stretch marks and pigmentation

Third trimester (weeks 27-40): final growth and preparing for labor

You are in the last stretch of the journey. The third trimester is the final stage of pregnancy, where the baby prepares for life outside the womb. Physical discomfort may increase as the due date approaches. For the baby this means:

• Rapid weight gain and fat accumulation
• The lungs mature, necessary for breathing oxygen
• Baby moves into head-down position for delivery
• Rapid development of the brain
• Eyes open and respond to light

Common symptoms for the mother during this stage:

• Difficulty sleeping
• Shortness of breath
• Frequent urination (due to womb pressing on bladder)

Baby is born!

Hopefully the delivery was not too challenging and the baby is healthy! We wish you and your partner a terrific time as family life begins. May your newborn be a blessing to you and the world.

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Estimating the due date follows a standardized method. The average duration of a human pregnancy is around 38 weeks as measured from conception and 40 weeks as measured from the last menstrual period (LMP) (Bar et al., 2025; Jukic et al., 2013; Pritschet et al., 2024). If you are actively trying to get pregnant however, it is often hard to know the exact conception date. It is easier to remember the last menstrual period. Often, sexually active women do a pregnancy test when the next expected menstrual period does not arrive. If the test is positive and the last menstrual period started on on October 12th, the estimated due date is 40 weeks later on July 19th. Another reason to do a pregnancy test not directly after intercourse is because the tests only get reliable roughly 10 days after conception. This is because, by then, a hormone called human chorionic gonadotropin (hCG) that the tests use to detect pregnancy becomes more reliably detectable.

It is important to know that usually pregnancy is measured from the first day of the last period. So, when we say someone is "6 weeks pregnant" it includes the two weeks between the first day of the last period and the actual fertilization of the egg.