Baby Size by Week: Complete Guide from 4 to 40 Weeks Pregnant

At some point in every pregnancy, someone hands you a chart comparing your baby to a piece of produce, and you nod and smile while internally wondering if the kumquat at week 10 is meant to be comforting or just deeply surreal.

But here's the thing — those size comparisons are actually useful. They give you a way to visualize something that's simultaneously the most concrete and most abstract thing in your life right now. This guide covers baby size at every week from 4 to 40, plus the stuff that actually matters: how your due date is calculated, what to do if your cycle isn't a tidy 28 days, and how IVF transfers (Day 3 and Day 5, fresh or frozen) change the math entirely.

How Your Due Date Is Actually Calculated

Before we get into the fruit-size runway show, it helps to understand where "how far along am I" comes from in the first place. Pregnancy is measured in weeks from your last menstrual period (LMP), not from conception — which confuses almost everyone initially, because you're technically "2 weeks pregnant" before you've even conceived.

The standard calculation — called Naegele's Rule — works like this:

If you know your conception date instead, you can still work backwards: subtract 14 days (the approximate time from LMP to ovulation in a 28-day cycle) to get your LMP equivalent, then add 280 days. The pregnancy calculator does this automatically if you select "Conception Date" as your input mode.

Want to know what week of pregnancy you are today, or how far along you are in weeks and days? The easiest way is to enter your LMP into the calculator — it'll give you weeks + days, trimester, days remaining, and a full milestone timeline instantly.

Due Date by Cycle Length: What If Your Cycle Isn't 28 Days?

Standard pregnancy calculators assume a 28-day cycle. For a lot of people, that's not reality. If you have a shorter or longer cycle, your ovulation timing shifts — and so does your due date.

The adjustment is simple: for every day your cycle differs from 28, your due date shifts by one day in the same direction.

Here's what that looks like across common cycle lengths:

Cycle Length	Ovulation Day	Due Date Shift	Notes
21 days	Day 7	−7 days earlier	Short cycle; early ovulation
24 days	Day 10	−4 days
26 days	Day 12	−2 days
28 days	Day 14	No adjustment	Standard calculator default
30 days	Day 16	+2 days
32 days	Day 18	+4 days
35 days	Day 21	+7 days later	Long cycle; late ovulation
40 days	Day 26	+12 days later	Very long cycle or irregular

The pregnancy calculator on this site has a cycle length field built in — you can dial it from 21 to 40 days and it'll adjust the due date automatically. No manual math required.

IVF Due Date Calculator: Day 3 Transfer vs Day 5 Transfer

IVF pregnancies don't have a "last period" that maps cleanly to a conception date in the traditional sense. Instead, the math works backwards from the embryo's age at transfer.

There are two common IVF embryo transfer types — and they give different due dates for the same transfer date:

IVF Day 3 Transfer (Cleavage-Stage Embryo)

A Day 3 embryo is 3 days old at transfer. In a natural cycle, the equivalent event would occur on day 17 (14 days to ovulation + 3 days of embryo development). So:

IVF Day 5 Transfer (Blastocyst)

A Day 5 blastocyst is 5 days old — 2 days further along than a Day 3. That shifts the LMP back by 2 more days:

Fresh vs Frozen Embryo Transfer

The IVF frozen embryo transfer calculator and fresh transfer calculator use the same math above — the Day 3 or Day 5 formula applies regardless of whether the embryo was fresh or previously frozen. The distinction is clinical (protocol, medications, timing of the cycle), not mathematical. Your due date is calculated identically once you know which day the embryo was transferred.

Baby Size by Week: The Complete Table (Weeks 4–40)

Through week 20, size is measured crown-to-rump (head to bottom). From week 21 onwards, it switches to crown-to-heel — which is why the numbers jump between weeks 20 and 21. The baby didn't teleport; the measuring tape just got longer.

Trimester breakdown: T1 = weeks 1–13   T2 = weeks 14–27   T3 = weeks 28–40

Week	Size Comparison	Length	Approx. Weight	Notes
4 wks	🌱 Poppy seed	~1 mm	—	Implantation complete; hCG rising
5 wks	🌿 Sesame seed	~2 mm	—	Heart tube forming
6 wks	🫛 Lentil	~4 mm	—	Heartbeat detectable by transvaginal ultrasound
7 wks	🫐 Blueberry	~1 cm	~1 g	Arm & leg buds visible; brain developing rapidly
8 wks	🍓 Raspberry	~1.6 cm	~1 g	Embryo → fetus; all major organs forming
9 wks	🍇 Grape	~2.3 cm	~2 g	Fingers and toes distinct; eyelids fusing
10 wks	🍊 Kumquat	~3.1 cm	~4 g	Bones hardening; swallowing reflex beginning
11 wks	🫐 Fig	~4.1 cm	~7 g	Hiccupping; genitals forming but not yet visible
12 wks	🍋 Lime	~5.4 cm	~14 g	End of highest-risk period for miscarriage
13 wks	🍑 Peach	~7.4 cm	~23 g	Last week of first trimester
14 wks	🍋 Lemon	~8.7 cm	~43 g	First trimester screening complete; second trimester begins
15 wks	🍎 Apple	~10.1 cm	~70 g	Ears developing; responds to sounds
16 wks	🥑 Avocado	~11.6 cm	~100 g	Facial expressions beginning; fetal movement increasing
17 wks	🍐 Pear	~13 cm	~140 g	Fingerprints forming; adipose tissue developing
18 wks	🫑 Bell pepper	~14.2 cm	~190 g	Quickening (first movements) often felt now
19 wks	🥭 Mango	~15.3 cm	~240 g	Vernix caseosa (protective coating) forming
20 wks	🍌 Banana	~16.4 cm (CRL) ~25.6 cm (CHL)	~300 g	Anatomy scan typically done; halfway point
21 wks	🥕 Carrot	~26.7 cm	~360 g	Measurement switches to crown-to-heel from here
22 wks	🫑 Spaghetti squash	~27.8 cm	~430 g	Eyelids still fused; hearing well-developed
23 wks	🍊 Grapefruit	~28.9 cm	~500 g	Approaching viability threshold
24 wks	🌽 Corn cob	~30 cm	~600 g	Viability milestone — lungs producing surfactant
25 wks	🥦 Cauliflower	~34.6 cm	~660 g	Nostrils beginning to open
26 wks	🌿 Scallion bundle	~35.6 cm	~760 g	Eyes begin opening; responds to light
27 wks	🥬 Head of lettuce	~36.6 cm	~875 g	Last week of second trimester
28 wks	🍆 Eggplant	~37.6 cm	~1.0 kg	Third trimester begins; brain growth accelerating
29 wks	🎃 Butternut squash	~38.6 cm	~1.15 kg	Kick counts recommended; REM sleep cycles begin
30 wks	🥬 Cabbage	~39.9 cm	~1.3 kg	Bone marrow taking over blood cell production
31 wks	🥥 Coconut	~41.1 cm	~1.5 kg	Rapid brain development; all 5 senses functional
32 wks	🫑 Bok choy	~42.4 cm	~1.7 kg	Fingernails and toenails forming; head may engage
33 wks	🍍 Pineapple	~43.7 cm	~1.9 kg	Immune system receiving antibodies from placenta
34 wks	🍈 Cantaloupe	~45 cm	~2.15 kg	Lungs nearly mature; late preterm threshold
35 wks	🍈 Honeydew melon	~46.2 cm	~2.4 kg	Brain still growing rapidly; most organs ready
36 wks	🥬 Romaine head	~47.4 cm	~2.6 kg	Weekly prenatal visits begin; baby considered early term at 37 wks
37 wks	🥦 Head of broccoli	~48.6 cm	~2.85 kg	Early term; most organs fully mature
38 wks	🧅 Leek	~49.8 cm	~3.1 kg	Full term begins; average birth weight range
39 wks	🍉 Mini watermelon	~50.7 cm	~3.3 kg	Full term; placenta beginning to age
40 wks	🎃 Pumpkin	~51.2 cm	~3.5 kg	Estimated due date — only ~5% of babies born exactly on EDD

Baby Size by Weeks and Days (Weeks 4–6)

When you're in the very early weeks, you might be looking for more granular information — like what size the baby is at 4 weeks 3 days, or 5 weeks 5 days. The honest answer is that growth at this stage is measured in fractions of a millimeter, so the differences between individual days are hard to detect even on ultrasound. But here's the approximate progression:

Week + Day	Approx. Length	What's Happening
4 weeks 1 day	~0.5 mm	Blastocyst fully implanted. The embryonic disc is forming — two cell layers that will become everything. hCG is rising but a pregnancy test may still be faint or negative at 4w1d–4w3d.
4 weeks 2 days	~0.7 mm
4 weeks 3 days	~0.9 mm
4 weeks 4 days	~1.1 mm
4 weeks 5 days	~1.3 mm
4 weeks 6 days	~1.5 mm
5 weeks 1 day	~2 mm	The primitive heart tube is beating. The amniotic sac and yolk sac are visible on transvaginal ultrasound. Most pregnancy tests will be clearly positive by 5 weeks.
5 weeks 2 days	~2.5 mm
5 weeks 3 days	~3 mm
5 weeks 4 days	~3.5 mm
5 weeks 5 days	~4 mm
5 weeks 6 days	~5 mm
6 weeks 1 day	~5.5 mm	A heartbeat is typically visible on transvaginal ultrasound at 6 weeks — usually between 100–120 bpm initially. The embryo has a distinct head pole and a curved "C" shape. Arm and leg buds are just beginning to form by 6w5–6w6.
6 weeks 2 days	~6 mm
6 weeks 3 days	~6.5 mm
6 weeks 4 days	~7 mm
6 weeks 5 days	~8 mm
6 weeks 6 days	~9 mm

Symptoms + Baby Growth by Week: The Honest Version

The fruit chart is great, but "avocado at 16 weeks" doesn't tell you much about how you're feeling. Here's a week-by-week breakdown of what's typically happening — both inside and out.

6 Weeks Pregnant

Baby size at 6 weeks pregnant: lentil, ~4 mm. Internally, the heart is beating. Externally, you probably feel nothing yet — or you feel everything. Nausea typically peaks between weeks 6–9 and is often worse in the morning (or, famously, whenever). Breast tenderness and fatigue are the two most consistent early symptoms. Some people have zero symptoms and panic; others have symptoms so severe they require medication. Both are normal.

7 Weeks Pregnant

Baby size at 7 weeks pregnant: blueberry, ~1 cm. The embryo's brain is developing at a rate of about 100 cells per minute. For the parent: nausea is probably in full swing. You may notice heightened sense of smell (which does not help with the nausea). Food aversions often kick in around now — particularly to meat and coffee, two things that are very difficult to avoid.

8 Weeks Pregnant

Baby size at 8 weeks pregnant: raspberry, ~1.6 cm. This is technically the last week of the embryonic period — next week, it becomes a fetus. All the major organ systems are forming simultaneously. Your uterus has doubled in size, though you almost certainly can't tell from the outside yet. Spotting after internal exams is common and usually harmless, but always worth mentioning to your provider.

9 Weeks Pregnant

Baby size at 9 weeks pregnant: grape, ~2.3 cm. Fingers and toes are distinct. The embryo officially becomes a fetus this week. For many people, this is when an early ultrasound happens and the heartbeat becomes real in a new way. Nausea often continues; fatigue remains significant. This is also when some people start telling close family — though statistically, the miscarriage risk has dropped noticeably from weeks 5–6.

10 Weeks Pregnant

Baby size at 10 weeks pregnant: kumquat, ~3.1 cm. The critical development period is wrapping up — most organ formation is complete, though everything still needs to grow significantly. NIPT (non-invasive prenatal testing) is now available if desired. Energy often starts to return slightly toward the end of the first trimester — though "slightly" is doing a lot of work in that sentence.

12 Weeks Pregnant

Baby size at 12 weeks pregnant: lime, ~5.4 cm. The risk of miscarriage drops significantly this week. Many people share the news publicly around 12–13 weeks. First trimester screening (nuchal translucency ultrasound + bloodwork) is usually done between 11–14 weeks. The baby can now yawn, hiccup, and make sucking motions — none of which you can feel yet, which feels like a missed opportunity.

20 Weeks Pregnant

Baby size at 20 weeks pregnant and movement: banana, ~16.4 cm (CRL). The anatomy scan happens around now — the big one, where you check all the major structures and can often find out the sex if desired. By week 20, most first-time parents have started to feel some movement, though it often feels like bubbles or flutters rather than kicks. Second-time parents usually feel it 2–3 weeks earlier.

Pregnancy Milestones: Week by Week Timeline

Here are the major milestones to watch for across all three trimesters, including when key developments happen and when to expect major appointments.

When Do Organs Develop During Pregnancy?

One of the most searched pregnancy questions is when do organs develop — and the answer spans the entire first trimester in a remarkably compressed timeline:

• Heart: Begins forming week 3–4; heartbeat detectable week 6
• Neural tube (brain + spinal cord): Forms and closes weeks 3–6 — this is why folic acid matters so much before pregnancy, since the window is often before you know you're pregnant
• Limbs: Arm buds appear week 5–6; fingers distinct by week 9
• Lungs: Begin forming week 4–5; functional surfactant production around week 24; fully mature ~week 36–37
• Kidneys: Begin functioning around week 9–11; start producing urine (which becomes amniotic fluid) around week 11–12
• Digestive system: Gut tube forming by week 6; small intestine functional by week 12; meconium forming by week 16
• Eyes: Optic cups forming week 4–5; eyelids forming week 9; eyes open around week 26–28
• Reproductive organs: Undifferentiated until week 7–8; external genitalia distinguishable on ultrasound around week 16–20

Frequently Asked Questions