Pregnancy week by week

medicinepregnancy

Hands on a pregnant belly. Photo: Suhyeon Choi, via Unsplash. CC0, via Wikimedia Commons.

Forty weeks, counted from a day that is not conception

Relative time counts from a moment instead of a calendar date. Here the moment is the first day of the last menstrual period, the day NHS and ACOG both date a pregnancy from, so “+12 w” means twelve completed weeks since then. The epoch is floating: every pregnancy has its own date for it, so this timeline shows none. Give it an anchored epoch, a real first day, and every week below turns into a calendar date.

Where the weeks come from: the milestones are the NHS’s week-by-week guide, week 4 to week 41, with the size comparison it gives for each. The trimesters, the test windows and the definitions of term are ACOG’s. They are typical figures and ranges, not a forecast for any one pregnancy.

What to try

Read down the first layer for the three trimesters, each with its weeks nested inside, and across the other layers for the scans, the tests and the names ACOG gives to the weeks around the due date.
Look at the two places where the sources differ: ACOG draws the second trimester from week 14 and NHS from week 13, and NHS calls a baby “full term” at week 37 where ACOG calls weeks 37 and 38 “early term”. The items are linked as contradicting.
Filter by tag: development, care or definitions.
Relative time also counts seconds: see A Falcon 9 launch.

Items

+0 d – +13 w
First trimester
3 months, 1 day
+4 w
Week 4: a poppy seed
+5 w
Week 5: a sesame seed and a first heartbeat
+6 w
Week 6: a pea
+7 w
Week 7: a grape and 100 new brain cells a minute
+8 w – +14 w
Dating scan window
1 month, 12 days
+8 w – +12 w
Booking appointment window
28 days
+8 w
Week 8: a raspberry, and an embryo becomes a foetus
+9 w
Week 9: a strawberry
+10 w
Week 10: an apricot with a heart at 180 beats a minute
+10 w
ACOG: an embryo becomes a fetus
+11 w
Week 11: a fig with fingernails
+12 w
Week 12: a plum
+13 w – +28 w
Second trimester
3 months, 15 days
+13 w
Week 13: a peach
+14 w
Week 14: a kiwi fruit
+15 w
Week 15: an apple
+16 w
Week 16: an avocado
+17 w
Week 17: a pomegranate
+18 w – +21 w
Anomaly scan window
21 days
+18 w
Week 18: a bell pepper
+19 w
Week 19: a beef tomato
+20 w
Week 20: a banana, and halfway
+21 w
Week 21: a carrot
+22 w
Week 22: a sweet potato
+23 w
Week 23: a large mango
+24 w – +28 w
Glucose screening window
28 days
+24 w
Week 24: corn on the cob, and viability
+25 w
Week 25: a courgette
+26 w
Week 26: a cucumber
+27 w
Week 27: a head of cauliflower
+28 w – +42 w
Third trimester
3 months, 8 days
+28 w
Week 28: an aubergine
+29 w
Week 29: a butternut squash
+30 w
Week 30: a cabbage
+31 w
Week 31: a coconut
+32 w
Week 32: a bunch of celery
+33 w
Week 33: a pineapple
+34 w
Week 34: a cantaloupe melon
+35 w
Week 35: a honeydew melon
+36 w – +38 w
Group B strep test window
14 days
+36 w
Week 36: a romaine lettuce
+37 w – +39 w
Early term
14 days
+37 w
Week 37: a leek, and “full term”
+38 w
Week 38: a stick of rhubarb
+39 w – +41 w
Full term
14 days
+39 w
Week 39: a small watermelon
+40 w
Week 40: a pumpkin, and the due date
+41 w – +42 w
Late term
7 days
+41 w
Week 41: a pumpkin or a watermelon