Pregnancy week by week: all 42 weeks

Forty-two pages, one per gestational week. Each opens with the figure the NHS publishes for that week, names the scans, tests and vaccinations offered around it, and ends with a single concrete thing a partner can do. Sources are on every page rather than in a footer.

The measurement basis changes at week 20, which is why the number appears to jump from 15.3 cm to 25.6 cm between weeks 19 and 20. Weeks 1, 2, 3, 41, 42 carry no length at all, because none is published for them.

First trimester (weeks 1-13)

Second trimester (weeks 14-27)

Third trimester (weeks 28-42)

Published length by gestational week, with the basis it was measured on
Week	Published figure	Measured	Comparison
1	not published	-	-
2	not published	-	-
3	not published	-	-
4	2 mm	head to bottom	about the size of a poppy seed
5	2 mm	head to bottom	about the size of a sesame seed
6	6 mm	head to bottom	about the size and shape of a pea
7	10 mm	head to bottom	about the size of a grape
8	16 mm	head to bottom	about the size of a raspberry
9	22 mm	head to bottom	about the size of a strawberry
10	30 mm	head to bottom	about the size of a small apricot
11	41 mm	head to bottom	about the size of a fig
12	5.4 cm	head to bottom	about the size of a plum
13	7.4 cm	head to bottom	about the size of a peach
14	8.5 cm	head to bottom	about the size of a kiwi fruit
15	10.1 cm	head to bottom	about the size of an apple
16	11.6 cm	head to bottom	about the size of an avocado
17	12 cm	head to bottom	about the size of a pomegranate
18	14.2 cm	head to bottom	about the size of a bell pepper
19	15.3 cm	head to bottom	about the size of a beef tomato
20	25.6 cm	head to heel	about the size of a banana
21	26.7 cm	head to toe	about the size of a carrot
22	27.8 cm	head to toe	about the size of a sweet potato
23	28.9 cm	head to heel	about the size of a large mango
24	30 cm	head to heel	about the size of a corn on the cob
25	34.6 cm	head to heel	about the size of a courgette
26	35.6 cm	head to heel	about the size of a cucumber
27	36.6 cm	head to heel	about the size of a head of cauliflower
28	37.6 cm	head to heel	about the size of an aubergine
29	38.6 cm	head to heel	about the size of a butternut squash
30	39.9 cm	head to heel	about the size of a cabbage
31	41.1 cm	head to heel	about the size and weight of a coconut
32	42.4 cm	head to heel	about the same length as a bunch of celery
33	43.7 cm	head to heel	about the size of a pineapple
34	45 cm	head to heel	about the size of a cantaloupe melon
35	46.2 cm	head to heel	about the size of a honeydew melon
36	47.4 cm	head to heel	about the size of a romaine lettuce
37	48.6 cm	head to heel	about the length of a leek
38	49.8 cm	head to heel	about the length of a stick of rhubarb
39	50.7 cm	head to heel	about the size of a watermelon
40	51.2 cm	head to heel	about the size of a pumpkin
41	3 to 4 kg (weight, no length published)	-	-
42	not published	-	-

The NHS publishes a length for each week from 4 to 40. Up to week 19 it is measured head to bottom (crown to rump, the measurement a scan takes while the legs are curled). From week 20 it is measured head to heel, which is why the figure jumps from 15.3 cm at week 19 to 25.6 cm at week 20 - the baby did not grow 10 cm in a week, the ruler changed. The NHS states the change on its week 20 page.

The rest of the wing

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The sources these weeks defer to

NHS, Week-by-week guide to pregnancy — week-by-week development, weeks 4 to 41
NHS: your antenatal appointments — the appointment schedule these pages follow
ACOG Committee Opinion 700: Methods for Estimating the Due Date — gestational age, the 280-day convention and the 37-42 week term window

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