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Trimester Guides

Your First Trimester: What to Expect Week by Week

Dr. Mary Richelle Montevirgen-Pascua

MD, FPOGS, FPSUOG · OB-GYN Ultrasound Subspecialist · 7 min read

The first trimester runs from the first day of your last menstrual period up to the end of week 13. So much happens in these early weeks — your baby grows from a tiny cluster of cells into a little one with a beating heart, arms and legs. Here is what to expect, and when an ultrasound can help.

Weeks 4 to 6: A positive test and the first signs

Most women find out they are pregnant around week 4 or 5, after a missed period. You may notice breast tenderness, tiredness, a heightened sense of smell or mild cramping. These are normal effects of rising pregnancy hormones.

By about 5 weeks, a transvaginal ultrasound can usually show the gestational sac inside the uterus. This confirms the pregnancy is in the right place and not ectopic.

Weeks 6 to 8: The first heartbeat

Between 6 and 7 weeks, the baby's heartbeat can often be seen flickering on ultrasound. This is one of the most reassuring moments of early pregnancy. Nausea and vomiting ("morning sickness") often peak around this time — eating small, frequent meals and staying hydrated can help.

Weeks 8 to 10: Accurate dating

An ultrasound in this window measures the baby from head to bottom (the crown-rump length). This is the most accurate way to set your due date, which guides the timing of every later scan and test.

Weeks 11 to 13: The first-trimester screening scan

Between 11 weeks and 13 weeks 6 days, a detailed scan can check the baby's early anatomy and measure the nuchal translucency — a small fluid space at the back of the neck. Together with your age and other factors, this helps estimate the chance of certain chromosomal conditions.

Taking care of yourself

  • Start or continue folic acid (at least 400 mcg daily) to help protect your baby's brain and spine.
  • Avoid smoking, alcohol, and medicines not approved by your doctor.
  • Rest when you can — first-trimester tiredness is real.
  • Schedule your first prenatal check-up and ultrasound.

Every pregnancy is different. If something feels unusual or you have questions, talk to your OB-GYN — no question is too small.

This article is for general education and does not replace advice from your doctor.

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