At 39 weeks your baby is about 49.5 cm and 3.3 kg — roughly a small pumpkin — fully grown and, quite literally, waiting on a hormonal signal to begin the process of labour.
The short answer: baby is fully grown now, with space so tight that movement feels like rolling rather than kicking, and lungs still producing surfactant for that first strong cry. For you, the cervix begins softening and effacing in preparation, and a sudden burst of energy — often called nesting — is common. It is worth pacing yourself even if the urge to clean and prepare everything is strong.
Your baby this week
Baby is now fully grown, and space has become so tight that movement feels different — more of a rolling sensation than the sharp kicks of earlier weeks, though movement should still be happening regularly. The lungs continue producing more and more surfactant, right up until birth, so that the first breath — and the first cry — comes out strong and confident.
Right now, baby is essentially waiting on a hormonal signal exchanged between baby and the placenta that will set labour in motion — a process not fully understood, and not one that can be reliably predicted or hurried along from outside.
Your body this week
The cervix begins softening and effacing (thinning) around now — real, physical preparation for labour, even before any contractions are felt. Many people also notice a sudden burst of energy, often called the nesting instinct — a strong urge to clean, organise, and get everything ready. It is a genuine and widely reported feeling, though it is worth being careful not to overexert yourself with heavy cleaning or lifting this late in pregnancy.
Movement should still happen regularly even in these final days. If baby feels unusually quiet for an extended stretch, it is worth calling rather than assuming things have simply slowed down because space is tight.
Tests and appointments
Weekly appointments continue for most care pathways, checking baby's position, heart rate, and your blood pressure as the due date approaches.
✅ Worth doing this week
- Pace yourself with any nesting urge — the energy is real, but so is the risk of overdoing it
- Keep counting movements daily, even as the pattern stays subtle
- Go straight to the hospital if your water breaks, even with no contractions yet
- Keep your phone charged and your bag by the door — this really is any-day-now territory
- Review your birth plan one last time with whoever will be with you
When to call your doctor
Call the same day if: your water breaks, movement feels reduced or unusually quiet, contractions become regular and increasingly strong, or you notice bleeding. Also call for a severe headache with visual disturbance, sudden swelling of the face or hands, or a temperature above 38°C.
FAQ — 39 weeks pregnant
What actually triggers labour to start?
The precise trigger is still not fully understood, but it involves a cascade of hormonal signals between baby and the placenta that ultimately causes the cervix to soften and the uterus to begin regular, coordinated contractions. It cannot reliably be predicted or forced ahead of that signal, which is exactly why the countdown to a due date is an estimate rather than an appointment.
Is the "nesting instinct" a real thing?
Many people describe a sudden burst of energy and a strong urge to clean, organise, or prepare in the final days — commonly called nesting. It is a widely reported experience rather than a rigorously studied medical phenomenon, but it is real for those who feel it. The main caution is not to overexert yourself climbing, lifting, or scrubbing for hours at 39 weeks, however tempting it is.
What if my water breaks with no contractions yet?
If your waters break — even without any contractions following — contact your care provider or go to the hospital right away rather than waiting to see if contractions start on their own. This is different from Braxton Hicks or ordinary discharge, and it needs prompt assessment regardless of how you feel otherwise.
How big is the baby at 39 weeks?
About 49.5 cm and 3.3 kg — roughly a small pumpkin. Space is now so tight that movement feels more like rolling than kicking, and the lungs continue producing surfactant right up to birth so that first breath and cry are strong.
Week 39 at a glance
At 39 weeks your baby is about 49.5 cm and 3.3 kg, fully grown and waiting on the hormonal signal that starts labour, with lungs still producing surfactant for that first strong cry. For you, the cervix softens and effaces in preparation, and a nesting burst of energy is common — worth pacing rather than pushing through at full speed.
Inside Baby Novum: your birth plan, hospital bag checklist, and contraction timer stay ready to go, so the final days feel like waiting with a plan rather than waiting in the dark.