The short answer
21 weeks = 5 months
At 21 weeks pregnant, you are about 5 months pregnant — in month 5 of your pregnancy and in your second trimester.
Not sure of your week? Use the pregnancy week calculator to find it from your last period, due date or conception date.
The fetus is entering a phase of steady weight gain, and movements are becoming stronger and easier to notice.
About the size of a carrot
size comparison
26.7 cm
crown to heel (10.5 in)
360 g
average weight (12.7 oz)
The bone marrow is now the main site of blood cell production, taking over from the liver and spleen. The digestive system is absorbing small amounts of sugar from swallowed amniotic fluid. Eyelashes and eyebrows are filling in, and the fetus has settled into recognizable sleep and activity patterns.
Sizes and weights are averages — healthy babies vary widely, and your scan measurements are what count.
Movement often becomes unmistakable this week, and you may notice it most when you lie down in the evening. Stretch marks can begin to appear on the belly, thighs or breasts as skin expands. Varicose veins and spider veins are common and usually improve after birth.
Common symptoms this week
133
days to go
19
weeks to go
147
days so far
At 21 weeks 0 days you are 147 days into a 280-day pregnancy, which leaves 133 days — 19 weeks — until 40 weeks. That number drops by one for every day you move through this week. Remember the due date is an estimate: only about 1 in 20 babies is born on it, and most arrive within two weeks on either side.
Next milestone: 49 days until the third trimester (28 weeks 0 days).
A calendar month averages about 4.3 weeks — not 4 — so you can't just divide your week number by four. Instead, months of pregnancy are counted as uneven buckets of 4 to 5 weeks, arranged so the full 40 weeks fits into 9 calendar months. Week 21 falls in month 5, in the second trimester.
Your care team will always use weeks and days rather than months, because screening windows and term definitions are set by exact week — months are just the everyday shorthand. For the full conversion, see the weeks to months chart, or go the other way with the months-to-weeks guide.
Your notes will show a week and a day — 21+3 or 21w3d, for example. Pick the exact day you are on for its own day count, countdown and month answer.
| Week + day | Shorthand | Days pregnant | Days to go |
|---|---|---|---|
| 21 weeks 0 days (this page) | 21+0 | 147 | 133 |
| 21 weeks 1 day | 21+1 | 148 | 132 |
| 21 weeks 2 days | 21+2 | 149 | 131 |
| 21 weeks 3 days | 21+3 | 150 | 130 |
| 21 weeks 4 days | 21+4 | 151 | 129 |
| 21 weeks 5 days | 21+5 | 152 | 128 |
| 21 weeks 6 days | 21+6 | 153 | 127 |
At 21 weeks pregnant, you are about 5 months pregnant — in month 5 of your pregnancy and in your second trimester. Months of pregnancy are counted as uneven 4–5 week buckets so that the full 40 weeks fits into 9 calendar months.
21 weeks is in the second trimester, which runs from 14 weeks 0 days through 27 weeks 6 days. The third trimester begins at 28 weeks 0 days.
About 19 weeks until the 40-week mark — though only about 1 in 20 babies arrives exactly on the due date. Most are born within two weeks on either side of it.
At 21 weeks 0 days you are 147 days into a 280-day pregnancy, so there are 133 days left until the 40-week mark — that's 19 weeks. The count drops by one for each day you move through week 21.
21 weeks is 147 days, counted from the first day of your last menstrual period (21 × 7 = 147). Add the days into your current week for the exact figure — for example 21 weeks 3 days is 150 days.