Week 18 opens the window for the most detailed scan of the pregnancy: the anatomy scan, offered between 18 and 22 weeks. Your baby is about 22.5 cm and 205 grams — sweet-potato-sized — and moving with noticeably more strength and precision than a few weeks ago.
The short answer: the anatomy scan window opens now and runs to 22 weeks, checking the brain, heart, spine, kidneys and limbs in detail. Nerve fibres are gaining a protective coating called myelin, which sharpens movements. The ears are fully formed and picking up the sounds of your own body. For you, dizziness on standing is common as blood vessels widen, and appetite is climbing as demand increases.
Your baby this week
The nervous system is undergoing myelination — nerve fibres throughout the body are being wrapped in a fatty insulating layer that dramatically speeds up how fast electrical signals travel from the brain to the muscles. This is a mechanical reason, not just a growth one, why movements this week and in the coming weeks become noticeably stronger and more coordinated: yawning, hiccupping and even grimacing are all now possible.
The ears are fully formed and functioning, and the fetus is listening closely to the sounds inside your body — your heartbeat and the rush of blood through your vessels are the loudest and clearest signals available at this stage, well before external sound becomes distinct.
Your body this week
Your uterus is now roughly two finger-widths below the navel — an easy landmark many people check for themselves between appointments. Blood vessels are actively dilating and your blood volume continues rising, which together explain orthostatic hypotension: the brief head-spin some women feel on standing up quickly. It usually passes in seconds; standing up in stages helps.
Your heart is working substantially harder than before pregnancy, with cardiac output continuing to climb toward its second-trimester peak. Appetite often grows noticeably around now too, tracking the baby's own accelerating energy demands rather than indicating anything is wrong.
Tests and appointments
Tests and scans around now: The detailed anatomy (anomaly) scan is usually offered between 18 and 22 weeks.
Exact timing varies by country and by your own care plan — your doctor or midwife will confirm the schedule that applies to you.
If the anatomy scan is not already booked, this is the week to arrange it. The sonographer works through a structured checklist covering the brain, face, spine, heart, stomach, kidneys, bladder, abdominal wall and all four limbs, and also confirms the placenta's position. It typically takes 20 to 30 minutes.
✅ Worth doing this week
- Book the anatomy scan if it is not already arranged, ideally early in the 18–22 week window
- Stand up in stages — from lying to sitting, then sitting to standing — if dizziness has started
- Ask about a ferritin check if it was not already done, as iron demand rises with blood volume
- Start easing off your back for long periods of lying down, gradually, ahead of the third trimester
- Eat to appetite rather than restricting; the growth curve genuinely steepens now
When to call your doctor
Call the same day if: you have bleeding, severe or one-sided abdominal pain, a severe headache with visual disturbance or sudden swelling of the face and hands, fainting (rather than brief light-headedness), a temperature above 38°C, or fluid leaking from the vagina.
FAQ — 18 weeks pregnant
When exactly should the anatomy scan be booked?
Between 18 and 22 weeks, and most units aim for the middle of that window — around 20 weeks — because the baby is large enough to see clearly but still has room to move into different positions during the appointment. Book as early in the window as your clinic allows; if a structure is hard to see because of the baby's position, there is more time left for a repeat visit.
Why do my baby's movements feel different this week?
The nervous system is undergoing myelination — nerve fibres are being coated in a fatty insulating layer called myelin, which lets electrical signals travel far faster between the brain and the muscles. That is a real mechanical reason movements become stronger and sharper around now, not just a matter of the baby getting bigger.
Is dizziness on standing normal at 18 weeks?
Yes, and it has a name: orthostatic hypotension. Widened blood vessels and a still-rising blood volume mean your blood pressure can dip briefly when you stand up quickly, before it catches up. Standing up slowly and avoiding long periods on your feet in heat both help. Dizziness with chest pain, palpitations, or an actual faint is different and needs assessing.
How big is the baby at 18 weeks?
About 22.5 cm and 205 grams — roughly a sweet potato. Growth has become steadier now, with proportionally more of it happening in length than in dramatic weekly leaps.
Week 18 at a glance
At 18 weeks your baby is about 22.5 cm and 205 grams, with nerve fibres gaining a myelin coating that makes movements stronger and sharper, fully formed ears already tuned to the sounds of your body, and the first yawns and hiccups. The anatomy scan window opens now and runs to 22 weeks. For you, brief dizziness on standing is common as blood vessels widen, the heart works harder as cardiac output climbs, and appetite tends to grow with it.
Inside Baby Novum: the anatomy scan window is calculated from your own dates and flagged before it closes, and the ultrasound journal stores each measurement and image together so nothing gets lost between appointments.