Average fetal length and weight chart

Find out how big your baby is during each week of pregnancy using our fetal growth chart. Babies grow at different rates, so this guide tells you the length and weight of an average baby from eight weeks of pregnancy, through to birth. You can expect your baby to grow most rapidly in the third trimester. By 40 weeks, the average baby weighs about 3.6kg (8.0lb), and is about 51.0cm (20.1in) long from head to heel. Your baby's growth will be checked by ultrasound at certain points in your pregnancy and their estimated fetal weight and size may vary from these averages. Your midwife will keep track of your baby's growth and will offer extra scans if you need them.

How and when will my baby be measured during pregnancy?

During your dating scan at around 12 weeks, your sonographer will measure your baby from their head to their bottom. This is called the crown-rump length (CRL), and it will be used to work out your due date (NHS England 2022a).

At your anomaly scan, which you’ll have at around 20 weeks, your baby will be more active, meaning CRL is not as accurate.

Instead, your sonographer will check:

• your baby’s head circumference (HC), which means the size of their head
• abdominal circumference (AC), which means the size of their body around their tummy
• femur length (FL), which means the length of their thigh bone
  (NHS England 2022b)

From about 24 weeks, your midwife will regularly check your baby’s growth by measuring the distance between your pubic bone and the top of your womb (fundal height) (NICE 2021a). They will offer you an extra ultrasound scan if there are signs that your baby is either small for dates or large for gestational age (NICE 2021a).

Using our fetal growth chart

The numbers in our chart below can give you an idea of your baby's size during each week of your pregnancy. Bear in mind that babies grow at different rates after the first trimester (Grantz et al 2016) so your baby may be smaller or larger than these averages. Every baby is different. Lots of factors affect how big your baby grows, including your height, weight (BMI), ethnicity and whether you’ve had a baby before (RCOG 2014, Romero and Tarca 2018). Some hospitals use individualised growth charts that take these factors into account (Gardosi et al 2018, Nicolaides et al 2018). These charts help midwives and doctors to identify babies who are growing too slowly or too fast more accurately and to offer extra care in good time.

Boys' measurements are also different to girls' measurements. They tend to be longer and heavier than girls (Kiserud et al 2017, Villar et al 2014, 2016). To keep things simple, the numbers in our chart are an average of boys and girls. We use crown-rump length (CRL) measurements from eight weeks to 13 weeks, then crown-to-heel measurements until 41 weeks. A healthy birth weight spans from 2.5kg (5.5lb) to just under 4kg (8.8lb) (NICE 2019, 2020).

Pregnancy week	Length (inches)	Weight (ounces)	Length (cm)	Mass (g)
8 weeks	0.6in	0.7oz	1.6cm	20g
9 weeks	0.9in	1oz	2.3cm	27g
10 weeks	1.2in	1.2oz	3.1cm	35g
11 weeks	1.6in	1.6oz	4.1cm	45g
12 weeks	2.1in	2.1oz	5.4cm	58g
13 weeks	2.6in	2.6oz	6.7cm	73g

The lengths below are from crown to heel:

Pregnancy week	Length (inches)	Weight (ounces)	Length (cm)	Mass (g/kg)
14 weeks	5.8in	3.3oz	14.7cm	93g
15 weeks	6.6in	4.1oz	16.7cm	117g
16 weeks	7.3in	5.2oz	18.6cm	146g
17 weeks	8in	6.4oz	20.4cm	181g
18 weeks	8.7in	7.9oz	22.2cm	223g
19 weeks	9.5in	9.6oz	24cm	273g
20 weeks	10.1in	11.7oz	25.7cm	331g
21 weeks	10.8in	14.1oz	27.4cm	399g
22 weeks	11.4in	1.1lb	29cm	478g
23 weeks	12.1in	1.3lb	30.6cm	568g
24 weeks	12.7in	1.5lb	32.2cm	670g
25 weeks	13.3in	1.7lb	33.7cm	785g
26 weeks	13.8in	2lb	35.1cm	913g
27 weeks	14.4in	2.3lb	36.6cm	1.1kg
28 weeks	14.8in	2.7lb	37.6cm	1.2kg
29 weeks	15.5in	3lb	39.3cm	1.4kg
30 weeks	16in	3.4lb	40.5cm	1.6kg
31 weeks	16.5in	3.9lb	41.8cm	1.8kg
32 weeks	16.9in	4.3lb	43cm	2kg
33 weeks	17.4in	4.8lb	44.1cm	2.2kg
34 weeks	17.8in	5.2lb	45.3cm	2.4kg
35 weeks	18.2in	5.7lb	46.3cm	2.6kg
36 weeks	18.6in	6.2lb	47.3cm	2.8kg
37 weeks	19in	6.7lb	48.3cm	3kg
38 weeks	19.4in	7.1lb	49.3cm	3.2kg
39 weeks	19.7in	7.6lb	50.1cm	3.4kg
40 weeks	20.1in	8lb	51cm	3.6kg
41 weeks	20.4in	8.4lb	51.8cm	3.8kg

Thanks to Dr. Mark Curran, maternal-fetal medicine specialist, for his help preparing these charts (Curran 2019). Measurements have been rounded for ease of use. For more detailed information see BabyCenter: fetal length and weight week by weekOpens a new window

What happens if my baby’s weight is too low or too high during pregnancy?

If your sonographer thinks that your baby is smaller or larger than expected for your stage of pregnancy, you will be offered extra antenatal care with an obstetrician (NICE 2021b, RCOG 2014). Some babies are naturally smaller or larger than the average baby and perfectly healthy (RCOG 2014). However, when a baby's weight falls either side of the normal limits (percentiles) for growth, it may be due to a problem with their development or how much food they are getting through the placenta (Sovio et al 2021). You may need to have further scans to check your baby's growth and wellbeing (RCOG 2014).

Find out what it might mean if your baby is smaller or larger than average.

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.
• Have you got your baby's red book (personal child health record) yet? Find out how it is used to track your baby's growth after the birth.