8 Pregnancy Milestones by Trimester

Forty weeks, three trimesters, and a lot happening beneath the surface. Here are 8 key milestones spread across the pregnancy, using gestational weeks (counted from your last period).

First trimester (weeks 1-12)

1. Week 5-6: The heartbeat begins

The heart starts beating around week 5-6, generally at roughly double an adult’s resting rate.

2. Week 8: The embryo becomes a fetus

By around week 8-9, the developing baby is formally referred to as a fetus rather than an embryo, with basic facial features and limb buds already visible.

3. Week 12: Major organ formation completes

By the end of the first trimester, all major organs have formed (though many will continue developing), and miscarriage risk drops meaningfully compared to earlier weeks.

Second trimester (weeks 13-26), often called the “golden period”

4. Week 16-20: You may feel the baby move

Many people first feel fetal movement (“quickening”) in this window, sometimes later for a first pregnancy.

5. Week 18-20: The anatomy scan

A detailed anatomy ultrasound, usually scheduled between 18 and 20 weeks, checks physical development in depth and is often when parents learn the baby’s sex.

6. Week 24: Viability milestone

Around 23-24 weeks is generally cited as the current threshold of fetal viability outside the womb with intensive medical care, though outcomes vary significantly by individual circumstances.

Third trimester (weeks 27-40)

7. Week 28-32: Rapid weight gain and brain development

The baby gains weight quickly, sleep-wake cycles become more regular, and the lungs continue maturing.

8. Week 37-39: Full term

A pregnancy reaches early term at 37-38 weeks and full term at 39-40 weeks. Only about 5% of babies actually arrive on their exact calculated due date; most arrive within a couple weeks of it in either direction.

Track your own current week and due date with our pregnancy due date calculator, and see gestational age vs fetal age for the terminology behind these week counts.