What pregnancy week 4 means
Implantation and a positive test. “4 weeks pregnant” means 4 weeks have passed since the first day of the last menstrual period — the standard way pregnancy is dated. Conception usually happened about two weeks after that date, so the baby’s own age (fetal age) is roughly 2 weeks. Week 4 runs from 4 weeks 0 days to 4 weeks 6 days; on the following day the pregnancy is 5 weeks.
Your baby at 4 weeks
The cluster of cells that will become the baby has finished burrowing into the lining of the uterus. It is already organised into layers: one will form the nervous system and skin, another the heart, muscles and bones, and the third the digestive organs. The placenta and the amniotic sac are beginning to form around it.
A common way to picture it: the baby is now about the size of a poppy seed — roughly about 1 mm. These are averages; babies grow at different rates, and ultrasound measurements vary.
Your body at 4 weeks
Four weeks is around the time a period would be due, so a missed period is often the first sign. The pregnancy hormone hCG is now rising quickly, which is why many home pregnancy tests turn positive this week. Some people notice light spotting, mild cramps, tiredness or tender breasts; others feel nothing different at all.
Care and appointments around week 4
If your test is positive, this is a good time to contact a doctor or midwife to arrange your first appointment, and to ask about folic acid if you are not already taking a prenatal supplement.
The timing of tests and appointments differs between countries and care providers. If anything worries you — such as bleeding, severe pain, fluid leaking, fever — contact your midwife, doctor or maternity unit straight away rather than waiting for your next appointment.
Week 4 in your pregnancy timeline
Week 4 is in the first trimester (weeks 1–13) and in pregnancy month 1, which covers roughly weeks 0–4. The second trimester begins in 10 weeks.
| Pregnancy start (first day of last period) | |
|---|---|
| Estimated conception | |
| Second trimester begins (14 weeks) | |
| Third trimester begins (28 weeks) | |
| Full term (39 weeks) | |
| Estimated due date (40 weeks) |
Dates assume you are exactly 4 weeks 0 days today. For dates based on your own last period, use the calculator above or the main Pregnancy Calculator.
4 weeks pregnant in days and months
4 weeks is 28 days of pregnancy — 10% of a 40-week pregnancy. Counted in calendar months, 4 weeks is about 0.9 months, which is usually described as month 1 of pregnancy. Months are only a rough grouping; healthcare professionals almost always talk in weeks and days because they are more precise. See Pregnancy Month Calculator for how weeks map to months.