Pregnancy Week-by-Week: A Complete Guide
Forty weeks, walked through one at a time — what's happening with your baby and your body, with the tools you'll need at each stage.
Pregnancy isn't a single experience — it's three trimesters with very different feelings, milestones, and concerns. This guide walks through all 40 weeks and tells you what's happening week by week.
How weeks are counted: by convention, pregnancy is dated from the first day of your last menstrual period (LMP), not from conception. So you're 'two weeks pregnant' before fertilization actually happens. Most people don't know they're pregnant until around week 4-5, when the home test turns positive.
Each week below has a baby-size comparison and one notable thing happening. As individual deep-dive pages ship, they'll link from this table — meanwhile this is a quick scannable reference.
Quick reference
First Trimester (Weeks 1–13)
13 weeksWeeks 1-13. Implantation, organ formation, and the most rapid development period. Morning sickness peaks around weeks 7-10 and eases by the end of this trimester.
| Stage | What's happening |
|---|---|
| Week 1 | Pre-conception — your last period starts the clock. |
| Week 2 | Ovulation around now. Fertilization may happen this week. |
| Week 3 | Conception complete — embryo is a tiny cluster of cells. |
| Week 4 | Implantation. Most home pregnancy tests turn positive now. |
| Week 5 | Size of a sesame seed. Heart begins to form. |
| Week 6 | Size of a lentil. First heartbeat detectable on ultrasound. |
| Week 7 | Size of a blueberry. Brain dividing into regions. |
| Week 8 | Size of a raspberry. Tiny fingers and toes forming. |
| Week 9 | Size of a grape. Basic body shape established. |
| Week 10 | Size of a kumquat. Vital organs are functioning. |
| Week 11 | Size of a fig. Baby can hiccup, even if you can't feel it. |
| Week 12 | Size of a lime. NT scan often happens this week. |
| Week 13 | Size of a peach. End of first trimester — morning sickness often eases. |
Second Trimester (Weeks 14–27)
14 weeksWeeks 14-27. Often called the 'honeymoon' trimester. Energy returns, morning sickness fades, and around week 18-20 you'll feel the first kicks.
| Stage | What's happening |
|---|---|
| Week 14 | Size of a lemon. Baby can squint, frown, and grimace. |
| Week 15 | Size of an apple. Baby can hear your voice. |
| Week 16 | Size of an avocado. Baby's eyes start moving. |
| Week 17 | Size of a pear. Skeleton hardening from cartilage to bone. |
| Week 18 | Size of a bell pepper. You may feel first kicks ('quickening'). |
| Week 19 | Size of a mango. Vernix forms to protect baby's skin. |
| Week 20 | Size of a banana. Halfway! Anatomy scan typically this week. |
| Week 21 | Size of a carrot. Baby's movements become more coordinated. |
| Week 22 | Size of a papaya. Tiny eyebrows forming. |
| Week 23 | Size of a grapefruit. Baby's hearing is well-developed. |
| Week 24 | Size of an ear of corn. Viability threshold — major NICU milestone. |
| Week 25 | Size of a head of cauliflower. Baby responds to your voice. |
| Week 26 | Size of a head of lettuce. Eyes start to open. |
| Week 27 | End of second trimester. Baby has regular sleep cycles. |
Third Trimester (Weeks 28–40)
13 weeksWeeks 28-40. The home stretch. Get to know your baby's usual pattern of movements, and call straight away if it changes. Hospital bag packed by week 36.
| Stage | What's happening |
|---|---|
| Week 28 | Size of an eggplant. Learn your baby's usual movements; call your midwife or doctor at once if they change. |
| Week 29 | Size of a butternut squash. Brain billions of neurons developed. |
| Week 30 | Size of a large cucumber. Baby practices breathing. |
| Week 31 | Size of a coconut. All major organs nearly fully developed. |
| Week 32 | Size of a squash. Time to start packing your hospital bag. |
| Week 33 | Size of a pineapple. Bones harden except the skull. |
| Week 34 | Size of a cantaloupe. Lungs nearly ready for the outside world. |
| Week 35 | Size of a honeydew melon. Group B Strep test typically this week. |
| Week 36 | Size of a romaine lettuce. Hospital bag must be ready. |
| Week 37 | Size of a Swiss chard bunch. Early term — brain and lungs still maturing. |
| Week 38 | Size of a leek. Most growth complete; gaining fat. |
| Week 39 | Size of a mini watermelon. Full term — labor can start any day. |
| Week 40 | Estimated due date. About 4% of babies arrive today (Jukic 2013). |
Frequently asked questions
Pregnancy is dated from the first day of your last menstrual period (LMP), not from conception. So 'four weeks pregnant' really means about two weeks since conception. This is the standard clinical convention because LMP is easier to date precisely.
End of week 13 marks the end of the first trimester. Some sources say end of week 12; the difference depends on whether they count completed weeks or current week. ACOG uses end of week 13.
The NHS says most people start to feel their baby move between 16 and 24 weeks, often as a gentle swirling or fluttering. In a first pregnancy you might not notice anything until after 20 weeks. If you have felt no movements by 24 weeks, tell your midwife or doctor.
Not to a set number. The NHS and the Royal College of Obstetricians and Gynaecologists (RCOG) say there is no number of movements that counts as normal. Instead, get to know your baby's usual pattern of movements. If the movements slow down, change or stop, contact your midwife or maternity unit straight away — don't wait until the next day, even at night. Some care providers ask you to count kicks; if yours does, follow their advice.
Some milestones to circle: week 8-13 (first prenatal + NT scan), week 18-22 (anatomy scan), week 24 (viability threshold), week 28 (start of the third trimester), week 36 (hospital bag ready), week 37 (early term).
Fruits are everyday objects in roughly the right size range for most weeks. The convention started in pregnancy books in the 1990s and stuck. Sizes are approximate — every baby develops on a slightly different curve.