The fetal period runs from week 9 until birth and is mostly about growth and maturing of organs that already exist. Pregnancy is counted from the last menstrual period and split into three trimesters.
First trimester (weeks 1 to 13)
Organs form and the fetus reaches about 7 cm by the end. The heartbeat can be seen on ultrasound by about week 6 and heard with a Doppler by about weeks 10 to 12.
Second trimester (weeks 14 to 27)
The fetus grows fast, and the mother usually feels movement between weeks 18 and 22. The lungs begin making surfactant, so survival outside the womb becomes possible near week 24.
Third trimester (weeks 28 to 40)
The fetus gains most of its weight as fat, and the lungs and brain mature. Most fetuses turn head down by week 36.
| Weeks | Milestone | What you can check |
|---|---|---|
| About 10 to 12 | Heartbeat heard by Doppler | Fetal heart rate 110 to 160 per minute |
| About 20 | Fundus at the umbilicus | Fundal height starts to match the weeks |
| About 24 | Threshold of viability | Lung maturity becomes key |
| 37 to 41 | Term | Position and presentation |
At the bedside
After 20 weeks, fundal height in centimeters roughly equals the weeks of pregnancy; a gap of more than 2 to 3 cm calls for an ultrasound. A normal fetal heart rate is 110 to 160 beats per minute.
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Written by Making Synapses. Topic reference: OpenStax, Anatomy and Physiology 2e (no text adapted).