Pregnancy Week Guide

Understand how pregnancy weeks are counted, what each trimester means, and the most important milestones for expectant parents. This guide is designed to support the calculator with useful medical context and practical planning advice.

How pregnancy weeks are calculated

Pregnancy is measured from the first day of your last menstrual period (LMP), not from conception. That means the date on the tracker starts before fertilization happens, which is why people are usually already 4 weeks pregnant when they miss their first period.

Healthcare providers use this standard method because the exact moment of conception is rarely known. Your estimated due date is usually determined by adding 280 days (40 weeks) to your LMP, which is called Naegele's rule.

Why week-based tracking matters

It aligns with how doctors schedule scans, tests, and prenatal checkups.
It helps you understand which risks and symptoms are normal at each stage.
It makes it easier to follow medical advice for nutrition, exercise, and fetal monitoring.

📏 Understanding Baby Sizes: Length vs. Volume

When tracking baby development week-by-week, you might notice a sudden jump in size around Week 20 (from about 15 cm to over 25 cm). This is not an overnight growth spurt! Rather, it is a transition in how medical professionals measure fetal size:

Weeks 1–19 (Crown-to-Rump Length): Because the baby's legs are curled up against their body, they are measured from the top of the head (crown) to the bottom (rump).
Weeks 20–42 (Crown-to-Heel Length): Once the baby grows large enough and stretches out, doctors measure from the top of the head to the heel of the foot.

To prevent size confusion, our pregnancy tracker focuses on displacement/3D volume alongside length. For example, a banana matches the 25cm length of a Week 20 baby, but a zucchini has the matching 3D volume (or weight of ~300g) which represents their true bulk much better.

👶 Fetal Growth Reference & Volume Chart

Here is the standard clinical progression of baby size, weight, and volume week by week:

Week	Avg Length (cm)	Avg Weight (g)	Est. Volume (mL)	Food Comparison	Object Comparison
Week 4	0.1 cm	< 0.001 g	< 0.01 mL	Poppy Seed	Grain of Sand
Week 8	1.6 cm	1 g	1.5 mL	Raspberry	Glass Marble
Week 12	5.4 cm	14 g	20 mL	Plum	Standard Lipstick Tube
Week 16	11.6 cm	100 g	140 mL	Avocado	Standard Light Bulb
Week 20	25.6 cm	300 g	380 mL	Zucchini (Medium)	Soda Can (12 oz)
Week 24	30.0 cm	600 g	950 mL	Medium Eggplant	1-Liter Milk Carton
Week 28	37.6 cm	1,000 g	2,800 mL	Spaghetti Squash	Thick Oxford Dictionary
Week 32	42.4 cm	1,700 g	7,800 mL	Honeydew Melon (Large)	Standard Men's Shoebox
Week 36	47.4 cm	2,600 g	18,000 mL	Pumpkin (Small)	Small Gym Duffel Bag
Week 40	51.2 cm	3,500 g	42,000 mL	Giant Watermelon	Infant Car Seat
Week 42	53.0 cm	3,900 g	58,000 mL	Extra Large Jackfruit	Rolling Suitcase (21-inch)

Trimester milestones

First Trimester (Weeks 1–12)

This is when your baby's foundations are formed. The embryo develops major organs, the heart starts beating, and the placenta begins to grow. Many people experience nausea, fatigue, and mood swings during this time.

Second Trimester (Weeks 13–26)

Often called the "golden trimester," this is when morning sickness eases for many people. Your baby starts moving, and you may see the gender during the anatomy scan around week 20.

Third Trimester (Weeks 27–40)

Growth accelerates and your baby gains most of its weight. This period is focused on lung development, reflexes, and getting ready for birth. Expect more physical discomfort and a stronger sense of baby movements.

Important: This guide is informational only and is not medical advice. Always follow your doctor’s recommendations and contact your healthcare provider if you have any concerns.

Weekly milestones to watch

Week 8: Most major organs are in place and the first prenatal scan usually happens.
Week 12: The risk of miscarriage decreases significantly and the baby’s bones are forming.
Week 16–20: You may feel the first baby movements, called "quickening."
Week 20: The anomaly scan checks fetal anatomy and placenta position.
Week 24: Viability improves; the lungs continue maturing.
Week 28: Your doctor may check blood pressure and screen for gestational diabetes.
Week 36: Your care team will assess baby’s position and prepare your birth plan.

How to use the tracker

Use the Pregnancy Week Tracker tool to calculate your current pregnancy week from either your last period date or your due date. The result helps you understand which stage you're in, what baby development milestones to expect, and when key appointments should happen.

Once you know your current week, scroll through the week-by-week timeline on the main tracker page to get detailed insights about fetal growth and maternal symptoms.

When to contact your healthcare provider

Severe abdominal pain or cramping
Heavy vaginal bleeding or fluid leakage
Not feeling baby movements as expected after 28 weeks
Sudden swelling, headache, or visual disturbances in the third trimester
Signs of infection, fever, or unusual discharge

Frequently asked questions

When is the due date most accurate?

The due date is most accurate in the first trimester, especially when confirmed by a dating scan around week 8–12. After that, the estimate may shift if the baby appears smaller or larger than expected.

Can I change my pregnancy week if my period date was uncertain?

Yes. If you have an early ultrasound, the doctor may adjust the due date based on the baby’s size. The tracker can still help you by showing the approximate week range.