Twelve weeks is the last week of the first trimester in most reckonings, and it is the week the nuchal scan is most often done. It is also the single most accurate moment in the whole pregnancy for dating.
Your baby is about 55 mm and 12 grams — lime-sized — and organogenesis is finishing.
The short answer: the 12-week scan measures crown-rump length and nuchal translucency, and dating is at its most accurate now — to within about five days — because all fetuses grow at nearly the same rate in the first trimester. The uterus rises above the pelvic brim, which relieves pressure on the bladder. The placenta has taken over hormone production, so nausea usually begins to lift from here.
Your baby this week
The critical construction phase is finishing. All the major systems are in place, and from here they grow and refine rather than form. The fingers and toes are fully separated, and the hands are beginning to clench into fists.
The kidneys are working and excreting urine into the amniotic fluid — which the fetus then swallows, in a cycle that continues until birth. The liver has started producing bile and the pancreas is producing insulin.
First reflexes appear. If your abdomen is pressed, the fetus may move in response — although at 12 grams there is nothing for you to feel yet, and there will not be for another month or more.
Your body this week
The uterus has risen above the pelvic brim. The immediate consequence is welcome — less pressure on the bladder, so fewer trips to the bathroom. The less welcome one is that it now begins to press on the intestines, which slows digestion and is where much of second-trimester constipation and bloating starts. Our constipation guide covers what is safe.
The placenta has taken over hormone production completely. That handover is why nausea typically starts to fade this week or next and why energy begins returning — the two symptoms women most want back.
Your abdomen will start growing faster from here. Moisturising will not prevent stretch marks — that is largely genetic — but it does help with the itching that comes from skin stretching.
Tests and appointments
Tests and scans around now: The nuchal translucency scan is most often done at 12 weeks, and dating is at its most accurate in this window.
Exact timing varies by country and by your own care plan — your doctor or midwife will confirm the schedule that applies to you.
At the scan the sonographer measures crown-rump length to confirm your dates, measures nuchal translucency, checks that the pregnancy is viable, counts the number of fetuses, and looks at the basic anatomy that is visible this early. Combined with the blood test, this produces your screening probability.
Calcium matters from now: the cartilage skeleton is beginning to ossify into real bone. Hard cheese, yoghurt, sesame and almonds are practical sources.
✅ Worth doing this week
- Write down the crown-rump length and the revised due date from the scan — everything later is calculated from it
- Increase calcium as ossification begins — dairy, sesame, almonds, tinned fish with bones
- Add fibre and fluid before constipation starts, not after
- Moisturise for the itching, and know that stretch marks themselves are mostly genetic
- Ask about treatment if nausea has not eased rather than assuming you must wait
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, fainting, a temperature above 38°C, burning on passing urine, or persistent vomiting. Severe itching without a rash, particularly on the palms and soles, is worth reporting at any stage from now on — it can signal a liver condition of pregnancy.
FAQ — 12 weeks pregnant
Why is the 12-week scan the most accurate for dating?
Because in the first trimester every fetus grows at almost the same rate, so crown-rump length maps very tightly onto gestational age — accurate to within about five days. After the first trimester, genuine variation in size appears and the same measurement becomes a much weaker guide to dates. That is why a due date set at 12 weeks is rarely changed later, and why it overrides your last-period estimate if the two disagree.
How big is the baby at 12 weeks?
About 55 mm crown to rump and roughly 12 grams — lime-sized. The fingers and toes are fully separated and starting to clench into fists, and the first reflexes are appearing.
Can the 12-week scan tell the sex?
Not reliably. Some sonographers will offer a guess based on the angle of the genital tubercle, but it is a guess, and the error rate at 12 weeks is high enough that it is not usually reported. The anatomy scan at 18–22 weeks is where sex is normally determined by ultrasound; NIPT can determine it from a blood sample from 10 weeks.
When will morning sickness stop?
For most women it eases between weeks 12 and 14 and is largely gone by 16. The reason is hormonal rather than psychological: hCG peaks around week 11 then falls, and the placenta takes over hormone production. About one in five women has nausea beyond the first trimester, and a smaller number have it throughout — that is not a sign of anything wrong, but it is a reason to ask about treatment rather than assume you have to wait it out.
Week 12 at a glance
At 12 weeks the baby is about 55 mm and 12 grams, with separated fingers clenching into fists, working kidneys excreting urine into the amniotic fluid, a liver producing bile and a pancreas producing insulin. Organogenesis is finishing. This is the most accurate week of the entire pregnancy for dating, because all fetuses grow at nearly the same rate in the first trimester — a due date set now is rarely changed. The uterus rises above the pelvic brim, easing bladder pressure but slowing digestion, and the placenta has taken over hormone production, which is why nausea usually starts lifting from here.
Inside Baby Novum: the ultrasound journal keeps the crown-rump length, the nuchal measurement and a photo of the printed report against the date, and plots each new measurement on a growth chart so later scans are read as a trend rather than a single number.