Brown fat continues accumulating from last week, and the skeleton's progression from cartilage to bone continues steadily. This week brings one more layer of physical preparation — both literal and remarkable: your baby is now covered in vernix caseosa.
Most parents see vernix at birth — particularly in full-term babies, who often still carry traces of it in skin folds, behind the ears, and in the creases of the arms. Some people instinctively wipe it away; current evidence suggests leaving it in place or gently rubbing it into the skin is preferable. It absorbs naturally within hours and is not dirty — it is the product of months of careful biological preparation.
The foetus is now 140mm — a sweet potato — and has reached 190g. The movements that have been present since around weeks 14–16 are becoming stronger and more frequent. Some people begin to feel movement from the outside of the abdomen for the first time around now, though this is more commonly possible from weeks 20–22. The kicks are not yet forceful enough for a hand pressed against the abdomen to reliably detect them; that comes later.
The foetus's bones, while hardening progressively, are still pliable enough to flex and compress during birth. The skull bones in particular are designed to overlap slightly — a process called moulding — allowing the head to navigate the birth canal. This is why some babies are born with temporarily elongated or misshapen heads that return to a rounder shape within the first days of life.
The midwife explained vernix to me at the twenty-week scan and I immediately felt guilty for not knowing about it. All that time it had been there, this whole protective system already running, and I'd had no idea. I made a decision then not to wipe it off at birth. It stayed on her for hours. It felt important.
The 20-week anomaly scan — which is offered between weeks 18 and 20 on the NHS — is the most detailed clinical assessment of your pregnancy between the first trimester and birth. It is significantly more comprehensive than the 12-week dating scan and covers a much wider range of structures. Understanding what it involves, what it can and cannot detect, and how its findings are communicated allows you to approach it as an informed participant rather than an anxious observer.
Face — the profile, lips, nose, and palate are checked. Cleft lip (not always cleft palate, which is harder to visualise) may be detectable.
Spine — the full length of the spine is examined, vertebra by vertebra, to look for spina bifida and other neural tube conditions.
Heart — the four chambers and major blood vessels are examined. Congenital heart conditions are the most common major birth defect, and many (though not all) are detectable at this scan.
Abdominal wall and organs — the stomach, liver, kidneys, and bladder are all checked. Gastroschisis (an abdominal wall defect) and absent or poorly functioning kidneys may be visible.
Limbs — all four limbs and their major bones are measured and assessed.
Placenta and fluid — the position of the placenta is confirmed. Low-lying placenta at this stage may require further monitoring. Amniotic fluid levels are assessed.
We decided not to find out the sex. The sonographer was brilliant — she told us she could see but wouldn't say, and she turned the screen slightly at one point without making a thing of it. It felt completely respected. I'd been worried it would be awkward. It wasn't at all.
Week 18 continues the physical progression of recent weeks with a few things worth noting specifically. The uterus is now roughly the size of a cantaloupe melon and sits clearly above the navel — the fundal height is measurable externally and your midwife will begin tracking it at appointments from around now as a simple but effective indicator of foetal growth.
For people who haven't felt movement yet, week 18 is within the range where first movements commonly arrive for first-time parents. The window is broad — weeks 16 to 22 — and position of the placenta, body composition, and individual sensitivity all play a role. If you are approaching week 20 without any sensation of movement, mention it at the anomaly scan appointment; it is not automatically a concern, but it is worth checking.
Sleeping position — from around week 16–20, sleeping on the back for extended periods becomes less advisable, as the weight of the uterus can compress the inferior vena cava (the major vein returning blood from the lower body) and reduce circulation. Left-side sleeping is most recommended, though brief periods on the back are not dangerous. A pillow between the knees makes side-sleeping more comfortable.
Nose and gums — the increased blood volume continues to make the mucous membranes more sensitive. Nosebleeds and bleeding gums, if they haven't appeared before, often become more frequent around now. Both are normal; neither requires treatment unless persistent or heavy.
The anomaly scan tends to occupy significant emotional space in the days approaching it, and the experience of it — whether it happens this week or in the next couple of weeks — is one of the most emotionally complex of the whole pregnancy. More so, for many people, than the 12-week scan: the 12-week scan was primarily about viability and relief; the anomaly scan is about something more structural and therefore more permanent-feeling.
Many people also describe the anomaly scan as the most visually impressive: the baby is now large enough to see clearly, moving, recognisably formed, with a face and discernible features. For partners, who may have found earlier scans more abstract, this is often the moment the baby becomes fully real. For the person carrying the pregnancy, it can mark a shift too — from knowing there is a baby to seeing, clearly, that this is a specific, distinct, already-individual person.
The anomaly scan was nothing like I'd expected. I'd steeled myself for a clinical, anxious experience. Instead they showed us the face, the profile, the hands opening and closing. The sonographer said "there's a cheeky one" when the baby put their hand in front of their face. I laughed. I hadn't expected to laugh. It was the most I'd felt like a parent so far.
If the scan identifies something that requires follow-up, the immediate period — receiving the information, processing it before any further appointments — is typically the hardest. Take someone with you if at all possible. Ask as many questions as you need. Know that you do not have to make any decisions in the room. The support available — from your midwife team, from the antenatal diagnostic service, and from organisations like Antenatal Results and Choices (arc-uk.org) — is there specifically for this.
The themes of week 18 connect directly to these full topic guides.
The anomaly scan is one of the appointments where being present matters most. For many partners, it is the most visually and emotionally significant moment in the pregnancy so far: the baby is large enough to see clearly, to watch moving, to have a face that is recognisably a face. This is typically the moment of deepest engagement for partners who have found the earlier weeks more abstract.
Come prepared: know what is being checked, have decided together about the sex, and have had the brief but important conversation about what you would want to do if the scan identified a finding requiring follow-up. Not because it is likely — most anomaly scans are normal — but because having spoken about it means you are not navigating a significant decision in shock.
- Be at the appointment. If there is any flexibility in your schedule, prioritise this one. The anomaly scan is qualitatively different from the others — richer, more detailed, more emotional — and being there together matters.
- Let yourself be moved. Seeing the baby's face, profile, and movements at 18 weeks is extraordinary. Partners sometimes feel pressure to hold steady for the person who is anxious; you are also allowed to simply feel what you feel. A hand held while crying is not a distraction.
- Ask questions. Sonographers expect partners to be present and engaged. If you don't understand what you're seeing, ask. If you want them to explain a particular image or measurement, ask. This is your appointment too.
Make a note about vernix — and decide now whether you want it left on at birth. This sounds premature, but birth preferences are best thought through from a position of calm curiosity rather than in the intensity of labour or the immediate postpartum. Vernix is one of the things most people don't know to ask about, and many hospitals still have a default of wiping it away. If you want it left on or massaged in — which the evidence supports — putting it in your birth preferences now means it doesn't rely on you remembering to say so in the room.
Once the sonographer has completed their checks and given you the headline results, ask:
Placenta position matters: a low-lying placenta at 20 weeks is common (around 1 in 20 pregnancies) and often resolves by 32 weeks as the uterus grows. But it requires a follow-up scan to confirm, and if it doesn't resolve it affects birth planning significantly. Knowing this early allows for appropriate monitoring. The second part of the question gives the sonographer space to mention anything that didn't rise to the level of a formal finding but is worth noting — a small soft marker, a borderline measurement — that you would benefit from knowing rather than discovering at a later appointment.