4 weeks pregnant
The pregnancy test has just turned positive; the gestational sac is only 2–3 mm.
Week 4 is when a missed period is usually noticed and the pregnancy test turns positive. The fertilised egg has implanted in the wall of the womb, and the pregnancy hormone hCG is starting to rise rapidly.
This week’s ultrasound image
Expected ultrasound appearance and measurements at week 4 (illustrative, to-scale drawing).
Abbreviations on the screen
- GS
- Gestational sac diameter
- YS
- Yolk sac
- CRL
- Crown–rump length
- NT
- Nuchal translucency
- BPD
- Biparietal diameter (head width)
- HC
- Head circumference
- AC
- Abdominal circumference
- FL
- Femur (thigh bone) length
- EFW
- Estimated fetal weight
- FHR
- Fetal heart rate (beats per minute)
- GA
- Gestational age (weeks + days)
- UA PI
- Umbilical artery Doppler resistance (PI)
Gestational sac
3 mm
How big is your baby this week?
About the size of a poppy seed
What can be seen on the ultrasound?
A transvaginal ultrasound may show a tiny 2–3 mm gestational sac within the thickened lining of the womb (endometrium); often, however, nothing can be seen yet. At this stage, a blood beta-hCG test is the most reliable way to confirm pregnancy.
Your baby’s development
The embryo measures about 1 mm and is made up of three layers of cells. These layers will go on to form the brain, heart, bones and all the other organs. The first structures of the placenta also begin to develop.
Changes in your body
A missed period is the most obvious sign. You may notice breast tenderness, mild pulling sensations in the lower abdomen, tiredness and light spotting (implantation bleeding); many people, however, have no symptoms at all.
Check-ups at this stage
- Urine or blood (beta-hCG) pregnancy test
- Monitoring the rise in beta-hCG over 48 hours (if needed)
- Reviewing any medicines you take with your doctor
Tips for this week
- Start taking folic acid (usually 400 micrograms a day)
- Stop smoking and drinking alcohol; keep caffeine below 200 mg a day
- Note the date of your last period: your pregnancy week is calculated from it
When should you see your doctor?
Severe pain on one side of the lower abdomen, shoulder pain, dizziness, feeling faint or heavy bleeding may be signs of an ectopic pregnancy; seek emergency care without delay.
Growth by week
Values are averages; differences of a few days or a few millimetres between babies are normal.
| Pregnancy weeks | Crown–rump length (cm) | Weight (g) |
|---|---|---|
| Week 4 | — | — |
| Week 5 | — | — |
| Week 6 | 4 mm | — |
| Week 7 | 10 mm | — |
| Week 8 | 16 mm | 1 g |
| Week 9 | 23 mm | 2 g |
| Week 10 | 32 mm | 4 g |
| Week 11 | 43 mm | 8 g |
| Week 12 | 55 mm | 14 g |
| Week 13 | 68 mm | 23 g |
| Week 14 | 85 mm | 43 g |
| Week 15 | 100 mm | 70 g |
| Week 16 | 115 mm | 100 g |
| Week 17 | 128 mm | 140 g |
| Week 18 | 140 mm | 190 g |
| Week 19 | 152 mm | 240 g |
| Week 20 | 25.6 cm | 300 g |
| Week 21 | 26.7 cm | 360 g |
| Week 22 | 27.8 cm | 430 g |
| Week 23 | 28.9 cm | 501 g |
| Week 24 | 30 cm | 600 g |
| Week 25 | 31.9 cm | 660 g |
| Week 26 | 33.8 cm | 760 g |
| Week 27 | 35.6 cm | 875 g |
| Week 28 | 37.5 cm | 1005 g |
| Week 29 | 38.8 cm | 1153 g |
| Week 30 | 40 cm | 1319 g |
| Week 31 | 41.3 cm | 1502 g |
| Week 32 | 42.5 cm | 1702 g |
| Week 33 | 43.6 cm | 1918 g |
| Week 34 | 44.8 cm | 2146 g |
| Week 35 | 45.9 cm | 2383 g |
| Week 36 | 47 cm | 2622 g |
| Week 37 | 48 cm | 2859 g |
| Week 38 | 49 cm | 3083 g |
| Week 39 | 50 cm | 3288 g |
| Week 40 | 51 cm | 3462 g |
Frequently asked questions
Can a 4-week pregnancy be seen on ultrasound?
Usually not. The gestational sac generally becomes visible on transvaginal ultrasound around week 5, once beta-hCG reaches roughly 1,500–3,500 mIU/mL.
What should beta-hCG be at 4 weeks?
Values vary widely at this stage (roughly 5–426 mIU/mL). Rather than a single result, what matters is whether the level rises adequately over 48 hours.
Your pregnancy care at SoftCare Clinic
Book an appointment with our specialist doctors for your ultrasound check-up and any questions you may have.
The information on this page is for general guidance only and does not replace a medical examination. Measurements are average values; please discuss your own results with your doctor.
Medical content editor: Op. Dr. Şükrü Sanus