18 weeks pregnant
At 18 weeks you are in the second trimester, and your baby measures about 14 cm from crown to rump and weighs about 190 g, roughly the length of a capsicum. The morphology scan window opens this week and runs to 20+6, and it is the main anatomy check of the pregnancy.
Weighs about 190 g, like a block of chocolate. Averages from standard growth tables; your scan gives your baby's own numbers.
Your baby at 18 weeks
The ears stand out and the baby may hear your voice. The body is catching up with the head, the face looks human and movements are becoming coordinated.
How you might feel
Many people feel the first movements this week, and leg cramps, backache and a larger appetite are common.
The morphology scan window opens.
Call your maternity unit or GP today for bleeding, severe pain, or if you cannot keep fluids down. 13 HEALTH (13 43 25 84) in Queensland; 000 in an emergency.
What's due this week in Australia
Tell your employer (when you are ready)
There is no legal deadline to tell your employer, but protections start once they know: transfer to a safe job, no-safe-job leave and special maternity leave.
Morphology (anatomy) ultrasound
A detailed 30–60 minute scan of the baby’s organs, spine, heart and limbs, the placenta position and the cervix.
Coming up: Whooping cough (pertussis) vaccine
Free under the National Immunisation Program in every pregnancy.
Coming up: 20-week antenatal visit
Review of the morphology scan, blood pressure, the first fundal-height measurement and a chat about the baby’s movements.
Checklist for the week
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Questions about 18 weeks
How many months is 18 weeks pregnant?
About four months.
What trimester is 18 weeks?
Week 18 is in the second trimester. In the Australian guidelines Hello Bub follows, the first trimester runs to 13 weeks, the second from 14 to 27 and the third from 28 weeks.
What is the morphology scan?
A detailed ultrasound at 18 to 20 weeks that checks the baby's brain, face, spine, heart, kidneys, stomach and limbs, the placenta position and the amount of fluid. It is free through public hospital imaging.
Written by the Hello Bub team; medical schedule follows the Australian Pregnancy Care Guidelines; last reviewed 3 November 2026.