Baby size by week in cm

During pregnancy, you’ll see your bump get bigger as your baby develops inside your uterus (womb). By 40 weeks, the average baby weighs 3.5kg (7.6lb), and is about 51.2cm long from head to heel (Hill 2019).

But how big is your baby right now? We’ve put together this simple chart to tell you the length and weight of an average baby from eight weeks of pregnancy, through to birth.

Bear in mind that boys tend to be longer and heavier than girls (Cole et al 2011, Villar et al 2014, 2016), and that every baby is different. So don't worry if the measurements in your maternity notes are different from those listed below. Your baby’s growth will be checked during your pregnancy, including through these ultrasound scans:

• During your dating scan or nuchal translucency (NT) scan between 10 and 14 weeks. Your sonographer will measure your baby from her head to her bottom. This is called the crown-rump length (CRL), and it will be used to work out your due date (Mizia et al 2018, NCCWCH 2008, NHS 2018).
• At your anomaly scan, which you’ll have at around 20 weeks. Your baby will be more active, meaning CRL isn’t as accurate. Instead, your sonographer will check your baby’s head circumference (HC), abdominal circumference (AC) and thigh bone (femur) length (FL) (ASUM 2018, NHS 2018).

To keep things simple, our chart uses crown-rump length measurements from eight weeks to 19 weeks, then crown-to-heel measurements until 42 weeks.

(Hill 2019)

The lengths below are from crown to heel:

(Hill 2019)

More on your growing baby:

• See how big your baby is now in our fruit and veg comparison slideshow.
• Check out our week-by-week guide to fetal development.
• Learn what it may mean if your baby is smaller or larger than average.

References

ASUM. 2018. Guidelines for the performance of second (mid) trimester ultrasound. Australasian Society for Ultrasound in Pregnancy. www.asum.com.au [Accessed October 2019]

Cole TJ, Williams AF, Wright CM, et al. 2011. Revised birth centiles for weight, length and head circumference in the UK-WHO growth charts. Ann Hum Biol 38(1):7-11

Hill MA. 2019. Fetal development: length – weight table. University of New South Wales, Embryology. embryology.med.unsw.edu.au [Accessed October 2019]

Mizia K, Campbell Westerway S, Robertson M. 2018. Guidelines for the performance of first trimester ultrasound. Australasian J Ultrasound Med 21:179-82

NCCWCH. 2008. Antenatal care: routine care for the healthy pregnant woman. Updated February 2019. National Collaborating Centre for Women’s and Children’s Health, NICE clinical guideline, 62. London: RCOG Press. www.nice.org.uk [Accessed December 2018]

NHS. 2018. NHS Fetal Anomaly Screening Programme handbook. NHS, Fetal Anomaly Screening Programme. www.gov.uk [Accessed October 2019]

Villar J, Cheikh Ismail L, Victora CG, et al. 2014. International standards for newborn weight, length and head circumference by gestational age and sex: the newborn cross-sectional study of the INTERGROWTH-21st project. Lancet 384(9946):857-68

Villar J, Giuliani F, Fenton TR, et al. 2016. INTERGROWTH-21st very preterm size at birth reference charts. Lancet 387(10021):844-5

Megan Rive is a communication, content strategy and project delivery specialist. She was Babycenter editor for six years.

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