4th week: Implantation
- Weeks and days
- 3+0 to 3+6
- Month
- 1
- Trimester
- 1
This week, the fertilized egg implants in the uterus. The early embryo measures only 0.1 to 0.2 millimeters. Your body starts to produce hCG, and an early test can turn positive toward the end of the week.
Week 4 of 40
Calculate your weekHow big is the early embryo?
0.1 to 0.2 mm
The early embryo is barely visible to the naked eye. It implants in the lining of the uterus, and other cells form the amniotic sac and amniotic fluid.
Length according to the Carnegie stages of embryology. At 1.2 mm, a poppy seed is already five to ten times larger.
This week
Your baby
- In 84% of cases, implantation takes place on day 8, 9, or 10 after ovulation, that is, at around 3+1 to 3+3.
- The amniotic sac and amniotic fluid begin to form.
- There is no heart or neural tube yet; the beginnings of the heart only form at the end of the 5th week of pregnancy.
Your body
- Your period is not due yet; with a 28-day cycle, it is due only at the start of the 5th week of pregnancy.
- The median hCG level in the blood is 17.5 mIU/ml. An early test can turn positive, but a negative result is not yet reliable.
- Symptoms are rare. Half of pregnant women notice the first signs only at around 5+1.
What matters now
- Folic acid 400 µg daily. According to the German recommendation, 800 µg if you did not already start taking it 4 weeks before conception.
- Iodine 100 to 150 µg daily.
- From the day your period is expected, a test is more reliable.
Get checked right away: one-sided pain in the lower abdomen, bleeding with a positive test, pain in the tip of the shoulder, or dizziness. This can be a sign of an ectopic pregnancy.
On ultrasound
- 3+0 to 3+6
Nothing visible yet, the gestational sac is still too small.
- 4+4
Gestational sac was visible in all pregnant women from this point, in the 5th week of pregnancy.
In Germany, health insurance pays for an early ultrasound only if a disrupted early pregnancy is suspected.
Frequently asked questions about the 4th week of pregnancy
How big is the baby in the 4th week of pregnancy?
The early embryo measures 0.1 to 0.2 mm and is barely visible to the naked eye. At 1.2 mm, a poppy seed is already five to ten times larger.
Does a pregnancy test show anything in the 4th week of pregnancy?
An early test can turn positive toward the end of the week. Even on the day the period is expected, a test detects at most 90% of pregnancies, and one week later 97%.
Why is nothing visible on ultrasound yet?
That is normal. In one study, the gestational sac was visible in all pregnant women only from 4+4.
Which month is the 4th week of pregnancy?
The 1st month of pregnancy, in the 1st trimester. Figures such as 3+4 weeks also belong to the 4th week of pregnancy.
Sources
- Carnegie stage table, UNSW Embryology
- Wilcox AJ et al.: Time of implantation of the conceptus and loss of pregnancy, NEJM 1999;340:1796
- Wilcox AJ et al.: Natural limits of pregnancy testing in relation to the expected menstrual period, JAMA 2001;286:1759
- Sayle AE et al.: A prospective study of the onset of symptoms of pregnancy, J Clin Epidemiol 2002;55:676
- Bree RL et al.: Transvaginal sonography in the evaluation of normal early pregnancy, AJR 1989;153:75
- Beta-HCG, test directory with the table from the Roche Elecsys HCG+β package insert, Ulm University Hospital (Universitätsklinikum Ulm), as of 8 July 2020
- Alaze-Hagemann F et al.: Recommendations by the Healthy Start Network 2026 (Handlungsempfehlungen Netzwerk Gesund ins Leben), Geburtshilfe Frauenheilkd 2026;86(6):e60
- Weeks 1 to 4 of pregnancy, familienplanung.de, Federal Institute for Public Health (BIÖG), as of 13 June 2024
- Ectopic pregnancy, NHS, as of 19 August 2026