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

7 min read

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're beginning to look like a fully formed newborn.

Length	About 43.7cm crown to heel
Weight	Around 1.9 kilograms, like a large mango
Weekly weight gain	Approximately 200–250g per week
Breastfeeding helpline	National Breastfeeding Helpline: 0300 100 0212

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're 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 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 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.

When to get help

Some symptoms in pregnancy can be a sign of something serious and need checking quickly. This is who to contact, and how soon.

Call 999 if:

you're bleeding from your vagina and the bleeding is heavy (soaking a period pad soon after putting it on), or you also have severe pain in your tummy, pain in your shoulder, or feel sick, faint, dizzy or lose consciousness
you think your baby is coming now and you have a strong urge to push (ask for an ambulance)
you have severe difficulty breathing (you're gasping, choking or not able to get words out) or your chest feels tight or heavy
you have pain and swelling in one leg and you also feel short of breath or have chest pain. A blood clot in the leg can travel to your lungs, which can be life-threatening
you have possible signs of sepsis, such as breathing very fast, being confused or having slurred speech, blue, pale or blotchy skin, a rash that does not fade when you press it, or a very high or very low temperature. Being pregnant makes sepsis more likely
you do not feel you can keep yourself safe (a mental health emergency is as serious as a physical one)

Always call 999 if you think it's a life-threatening emergency, no matter what stage of pregnancy you are.

Call your midwife or maternity unit now if:

your baby is moving less than usual, you cannot feel your baby moving anymore, or there is a change to your baby's usual pattern of movements
you have light bleeding or spotting, even if you have no pain or only mild tummy pain
you have stomach pain that is severe or does not go away after you've rested for 30 to 60 minutes, or stomach pain with regular cramping or tightenings, lower back pain, or pain when you pee
you're less than 37 weeks pregnant and you have a gush or trickle of fluid from your vagina (this could be your waters breaking), regular contractions or tightenings, period-type pains, or backache that's not usual for you
your waters break, even if labour has not started
you have signs of pre-eclampsia: a severe headache that does not go away with simple painkillers, vision problems (such as blurred vision or seeing flashing lights), pain below the ribs, sudden swelling of the face, hands or feet, feeling very unwell, heartburn that does not go away with heartburn medicines, or being sick (vomiting)
your skin is very itchy (for example, it keeps you awake at night), the palms of your hands or soles of your feet are itchy, or the itching is worse at night. This can be a sign of a liver condition called intrahepatic cholestasis of pregnancy (ICP) that needs urgent treatment
your breathlessness comes on suddenly, you're out of breath when you're lying down, or you're breathless and also have chest pain or feel dizzy
you have pain or swelling in one leg, usually in the calf or thigh. This could be a blood clot (deep vein thrombosis, or DVT), which is more likely when you're pregnant
you have a high temperature (38C or above), or you feel hotter than usual or shivery. A high temperature can be a sign of an infection, so it's important to get it checked
any of your contractions last longer than 2 minutes, or you're having 6 or more contractions every 10 minutes

If your baby's movements change, do not wait until the next day: call immediately, even if it's the middle of the night. Do not use a home doppler to check the heartbeat yourself. Even if you hear a heartbeat, this does not mean your baby is well.

If your waters break, use a period pad (not a tampon) so your midwife can check the colour of the waters, and tell them immediately if the waters are smelly or coloured.

You can also call your midwife or maternity unit if you're unsure or worried about anything.

If you cannot reach them, or you're not sure what to do, call NHS 111.

Speak to your midwife or GP if:

you're worried about your mental health, or stress, anxiety or low mood are affecting your day-to-day life. If you need urgent help for your mental health, call 111 and select the mental health option, or ask for an urgent GP appointment

Go to sleep on your side. From 28 weeks, the safest position to go to sleep is on your side, either left or right. Research suggests that, after 28 weeks, falling asleep on your back can double the risk of stillbirth. This includes daytime naps. If you wake up on your back, just turn onto your side and go back to sleep.

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.

Sources & further reading

General information, not individual medical advice. Speak to your GP, midwife or health visitor about your baby’s needs.