See how your baby grows with this free baby development week calculator. It shows a size comparison and the key things developing at each week of pregnancy, from week 4 to 40.
Enter or slide to a week from 4 to 40 to see roughly how big the baby is (with a familiar size comparison) and what’s typically developing that week.
General educational information about typical development, not medical advice. Speak with your midwife or doctor about your baby and pregnancy.
How the Baby Development Week Calculator Works
This tool looks up a familiar size comparison and a short note on typical development for a chosen week of pregnancy, along with the trimester and how many weeks remain to 40. It draws from a standard week-by-week reference, not from any measurement of your own baby.
Formula: Each week from 4 to 40 maps to a fixed size comparison (for example, “a lemon” or “a small pumpkin”) and a short typical-development note, read directly from a reference table. Trimester is set as First for weeks 1–13, Second for weeks 14–27, and Third for weeks 28 and beyond. Weeks remaining is calculated as 40 minus the entered week.
- Week of pregnancy — the input, entered or dragged on the slider, from 4 to 40 weeks.
- Size comparison — a familiar object roughly matching the baby’s typical size that week.
- Typically developing now — a short note on what’s generally happening in development at that week.
- Trimester — First, Second, or Third, based on the entered week.
- Weeks to 40 — how many weeks remain until the week-40 due-date reference point.
Example Scenarios
| Week | Size Comparison | Trimester | Weeks Remaining to 40 |
|---|---|---|---|
| 8 | a kidney bean | First | 32 weeks |
| 14 | a lemon | Second | 26 weeks |
| 20 | a banana | Second | 20 weeks |
| 28 | an eggplant | Third | 12 weeks |
| 34 | a cantaloupe | Third | 6 weeks |
| 40 | a small pumpkin | Third | 0 weeks |
Reading the Baby Development Week Calculator the Right Way
Pregnancy weeks are counted from the first day of your last period, not from conception, so week 6 in a baby development week calculator describes an embryo about four weeks after conception.
- If an early scan changed your due date, use the week your maternity team gives you, not the one worked out from your period.
- Size comparisons are averages. Until around 20 weeks, length is usually measured crown to rump and after that crown to heel, which is why some charts seem to jump.
- Milestones are approximate; a feature listed in one week of a baby development week calculator may appear a week earlier or later elsewhere.
Your baby’s measured size on a scan can differ from the averages in a baby development week calculator without anything being wrong; your sonographer or midwife will say if growth needs checking.
For worries such as bleeding, pain or later changes in movements, contact your midwife or doctor rather than relying on a baby development week calculator. The prenatal development article on Wikipedia gives more detail on each stage the baby development week calculator covers.
Baby Development Week Calculator FAQ
What does the size comparison (like “a banana”) actually represent?
It’s a familiar, everyday object roughly matching the typical average length or size of a baby at that gestational week, meant to make the number easier to picture. It’s not a measurement of your specific baby, just a general week-by-week reference point.Are these developmental milestones exact, or do they vary by baby?
They describe typical development for that week in general terms, not exact events for every baby. Every baby develops at its own pace, so these notes are meant as general education rather than predictions about your pregnancy specifically.Why does the trimester change at week 14 and week 28 here?
This calculator defines First trimester as weeks 1–13, Second as weeks 14–27, and Third as week 28 onward, which is a commonly used way to divide pregnancy into thirds. Some sources use slightly different week cutoffs for each trimester.What if I don’t know my exact week of pregnancy?
Use the due date calculator or pregnancy week calculator to estimate your current week from your last period or due date, then enter that week here. Your provider’s dating ultrasound is the most reliable source if you’re unsure.Does “weeks to 40” mean I’ll deliver exactly then?
No. Week 40 is simply the standard full-term reference point used for counting down, not a guaranteed delivery date. Many healthy births happen before or after week 40, and your provider tracks your individual due date and progress.Is this calculator based on ultrasound measurements of my baby?
No. It uses a general reference table of typical sizes and milestones by week, not any scan of your baby. For information specific to your baby’s growth, an ultrasound with your midwife or doctor is the appropriate source.Related Calculators
Pair this with the fetal weight calculator to see typical average weight and length for the same week, or use the pregnancy week calculator and due date calculator to confirm exactly where you are in your pregnancy.