Baby Growth by Week Calculator | Pregnancy Size and Weight Tracker

Calculator inputs

Use any one method below. The calculator area uses a 3-column layout on large screens, 2 columns on medium screens, and 1 column on mobile.

Educational note: Weekly fetal size estimates vary by source, scan method, and individual pregnancy. This tool is for general tracking and planning, not diagnosis.

Example data table

Week	Length Estimate	Weight Estimate	Measurement Basis	Example Growth Stage
8	1.60 cm	2 g	Crown-rump	Heartbeat, limb buds, and major organ foundations are developing.
12	5.40 cm	14 g	Crown-rump	First trimester growth accelerates and facial features become clearer.
20	25.60 cm	300 g	Head-to-heel	Baby responds more to sound and anatomy details become clearer.
28	37.60 cm	1,005 g	Head-to-heel	Sleep cycles, movement, and steady weight gain increase.
36	47.40 cm	2,622 g	Head-to-heel	Practice breathing continues as the baby fills out.
40	51.20 cm	3,462 g	Head-to-heel	Final positioning and organ maturity continue toward birth.

Formula used

Gestational Age (days) = (Weeks × 7) + Extra Days

Gestational Age from LMP = Current Date - Last Menstrual Period Date

Gestational Age from Due Date = 280 - Days Until Due Date

Interpolated Value = Lower Week Value + Fraction × (Upper Week Value - Lower Week Value)

Adjusted Length = Reference Length × Profile Factor × (1 + Custom Adjustment ÷ 100)

Adjusted Weight = Reference Weight × Profile Factor × (1 + Custom Adjustment ÷ 100)

Estimated Weekly Gain = Current Adjusted Weight - Previous Week Adjusted Weight

Weeks Remaining = max(0, 280 - Gestational Age in Days) ÷ 7

This calculator uses weekly reference values, linearly interpolates between weeks for day-level estimates, and then applies the selected growth profile plus any custom adjustment.

How to use this calculator

Select how you want to calculate pregnancy timing.

Enter either the gestational week, the last menstrual period, or the due date.

Choose a growth profile that best matches the estimate range you want.

Apply an optional custom adjustment for a slightly smaller or larger estimate.

Select metric-first or imperial-first outputs.

Choose the graph style and press the calculate button.

Review the result card above the form for size, weight, trimester, and time-to-due-date details.

Use the CSV or PDF buttons to export the result and weekly growth series.

Frequently asked questions

1. Is this calculator a medical diagnostic tool?

No. It provides educational growth estimates for planning and comparison. Ultrasound measurements and clinician guidance are more appropriate for medical decisions.

2. Why does the length measurement style change later?

Early pregnancy length is often tracked as crown-rump length. Later pregnancy estimates are commonly discussed as head-to-heel length, so the display label changes with gestational age.

3. Why can two babies differ at the same week?

Every pregnancy is different. Genetics, maternal health, placental function, and measurement technique can all affect estimated fetal size and weight.

4. Which input method should I use?

Use direct week entry when you already know the gestational week. Use LMP or due date when you want the calculator to estimate the current week automatically.

5. What does the growth profile option do?

It shifts the estimate into a lower, average, or upper range. It does not replace ultrasound percentiles or customized clinical growth charts.

6. What does custom adjustment mean?

It adds a manual percentage change to the chosen growth profile. This is useful for planning scenarios, but it should not be treated as a clinical percentile.

7. Why does the due date use 40 weeks?

Pregnancy timelines are commonly estimated from a 40-week reference. The calculator uses that standard to compute due-date timing and progress percentages.

8. Can I export my results?

Yes. After calculating, use the CSV or PDF buttons to download the summary values and the full weekly growth series generated from your selected profile.