Week 27 marks the start of the third trimester for most care pathways. Your baby is about 36 cm and 1 kg — roughly a cauliflower, and the first week where weight crosses into kilograms — with eyes that have started to blink and lungs that keep maturing toward birth.
The short answer: the third trimester begins around now, and with it a shift in what gets monitored. Baby is blinking for the first time, faint light can be detected through the skin of the abdomen, and the brain's surface is starting to fold into its first gyri. For you, the uterus sits well above the navel, mild breathlessness and thirst are common, and daily blood pressure awareness starts to matter more as the risk of preeclampsia rises from this point on.
Your baby this week
The eyes are no longer just opening — blinking has started, a small but genuine motor milestone. It is still dark inside the uterus, but faint light filtering through the abdominal wall and skin can now be detected, even if nothing resembling an image forms. The lungs continue producing surfactant, the substance that keeps the tiny air sacs from collapsing after the first breath, and production keeps increasing right through to birth.
The brain is undergoing a visible structural change: its surface, smooth until now, is beginning to develop its first folds, called gyri. This folding dramatically increases the surface area available for neurons, supporting the intense neural growth of the coming weeks. Weight has just crossed the 1 kg mark, a rough milestone many parents notice because it is the point where the app and most tracking tools switch from grams to kilograms.
Your body this week
The uterus now sits well above the navel, and its upper edge is close enough to the diaphragm that a little pressure on it is common — this is one reason mild shortness of breath can start to show up, especially climbing stairs or after a meal. Metabolism is running faster to support the growing baby, so feeling thirstier than usual is normal, and keeping water within reach helps more than it might seem.
Ankle swelling by the end of the day is also common now as extra fluid volume and reduced circulation from the growing uterus pool in the lower legs. Propping the feet up when sitting and avoiding standing in one spot for long stretches both help. This is also the week many care pathways introduce daily blood pressure checks, since the risk of pregnancy-specific blood pressure conditions rises through the third trimester and catching a trend early makes a real difference to how it is managed.
Tests and appointments
Tests and scans around now: Last week of the 24-28 window for gestational diabetes screening.
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 glucose tolerance test has not been done yet, this is the last week of the usual 24–28 week window — worth booking now rather than letting it slip past the deadline. From here, appointments typically become more frequent, moving toward every two weeks through much of the third trimester.
✅ Worth doing this week
- Start daily blood pressure checks if your care provider has recommended a home monitor
- Book the glucose test now if it has not been scheduled yet — the window closes this week
- Practise relaxed breathing techniques — useful now for breathlessness and later for labour
- Keep water within reach through the day rather than catching up all at once
- Elevate your feet when sitting to ease evening ankle swelling
When to call your doctor
Call the same day if: you notice a severe or persistent headache, visual disturbances such as flashing lights or blurring, sudden swelling of the face or hands, pain just under the ribs on the right side, or a blood pressure reading well above your usual range — this combination needs same-day assessment for preeclampsia. Also call for reduced movement, bleeding, regular tightenings before 37 weeks, or a temperature above 38°C.
FAQ — 27 weeks pregnant
Is 27 weeks the start of the third trimester?
By the most common convention, yes — many care providers count the third trimester from 27 or 28 weeks through birth, though the exact week varies slightly by source since pregnancy is not naturally divided into three equal blocks. What matters more than the label is that the pace of appointments and monitoring changes from here, regardless of which week is used as the cutoff.
Why does blood pressure monitoring matter more now?
Preeclampsia, a blood-pressure condition specific to pregnancy, becomes more likely from the third trimester onward, and catching it early — before symptoms appear — is what daily or per-visit blood pressure checks are for. A single raised reading is not a diagnosis, but a pattern of rising numbers, especially with swelling or headache, is worth reporting promptly rather than waiting for the next scheduled appointment.
Is swelling in the ankles normal at 27 weeks?
Mild swelling by the end of the day, especially in warm weather or after standing, is common in the third trimester as extra fluid and pressure from the growing uterus affect circulation in the legs. Swelling that appears suddenly, is severe, involves the face and hands, or comes with a headache or visual changes is a different picture and needs same-day assessment for preeclampsia.
How big is the baby at 27 weeks?
About 36 cm and 1 kg — roughly a cauliflower, and the first week of pregnancy where weight is measured in kilograms rather than grams. The brain's surface is starting to develop its first folds, called gyri, a structural change that increases surface area for the rapid neural growth still to come.
Week 27 at a glance
At 27 weeks your baby is about 36 cm and 1 kg, blinking for the first time and detecting faint light through the skin of the abdomen, with a brain surface that is just starting to fold into its first gyri. For you, the third trimester begins around now: the uterus sits well above the navel, mild breathlessness and thirst are common, and daily blood pressure awareness starts to matter as the risk of preeclampsia rises from this point through the rest of pregnancy.
Inside Baby Novum: log blood pressure readings and swelling alongside kick counts from the third trimester on, so a trend worth flagging at your next appointment is something you can show, not just recall from memory.