Pregnancy Week 33: Nesting, Baby's Bones, and Getting Ready

Quick answer: At 33 weeks your baby measures about 43.7cm from crown to heel and weighs around 1.9 kilograms, gaining roughly 200–250g a week. Their bones are hardening through ossification, though the skull stays flexible to allow moulding during birth. Many women feel the nesting instinct now — go with it, but avoid heavy lifting, ladders and strong chemical cleaners, and save your energy for labour.

What parents need to know

Week 33 often brings a burst of energy and motivation to prepare your home for your baby's arrival — a phenomenon commonly known as the nesting instinct. While the urge to scrub skirting boards at midnight might feel intense, it's worth channelling this energy wisely. Meanwhile, your baby's bones are hardening and they are really beginning to look like a fully formed newborn.

How big is your baby at 33 weeks?

Your baby now measures approximately 43.7cm from crown to heel and weighs around 1.9 kilograms — roughly the weight of a large mango. They are continuing to gain approximately 200–250g per week.

What's happening with your baby?

Your baby's bones are continuing to harden through a process called ossification. The skull bones, however, remain somewhat flexible and are designed to overlap slightly during the birth process (known as moulding), which allows the baby's head to fit through the birth canal. This is why many newborns have an elongated head shape immediately after a vaginal birth — this rounds out within a few days.

Your baby's immune system is developing with the help of antibodies passed from you through the placenta. These antibodies will provide your baby with temporary protection against many common illnesses in the early weeks of life. Breastfeeding continues this transfer via colostrum and then breast milk, which is one of the reasons breastfeeding offers significant immune benefits to newborns.

The nesting instinct

The nesting instinct is a well-recognised phenomenon in late pregnancy, characterised by an overwhelming urge to clean, organise, and prepare the home. It's thought to be driven by hormonal changes and an instinctive drive to create a safe, clean environment for the new arrival. If you're experiencing nesting urges, it's fine to go with them — but with some sensible caveats. Avoid activities that involve heavy lifting, climbing ladders, or using strong chemical cleaning products. Don't exhaust yourself, as you need to conserve energy for labour. Prioritise the things that will genuinely matter after the baby arrives: preparing the baby's sleeping area, washing and putting away baby clothes, and ensuring essential supplies are ready.

Useful things to do in nesting mode include: washing all baby clothes and bedding in non-biological laundry detergent; setting up and testing any baby equipment; preparing and freezing meals for after the birth (new parents rarely have time or energy to cook in the early days); and creating a calm, welcoming space for coming home with your baby.

Preparing for breastfeeding

If you plan to breastfeed, now is a good time to prepare. Attend an NCT or NHS antenatal breastfeeding session if available in your area. Reading about breastfeeding and what to expect in the early days can help enormously — the reality is that breastfeeding can be challenging at first and many women need support. Find out what breastfeeding support is available locally: your community midwife, health visitor, local breastfeeding groups, and the National Breastfeeding Helpline (0300 100 0212) are all valuable resources.

Your week 33 checklist

This week:

• Channel nesting energy into sensible preparations.
• Wash baby clothes and bedding.
• Find out about breastfeeding support in your area.
• Prepare some freezer meals if possible.
• Continue sleeping on your side.