13th week: The second trimester begins

Weeks and days
12+0 to 12+6
Month
4
Trimester
2

The 2nd trimester begins with the 13th week of pregnancy. This week, the fetus grows from about 54 to 74 millimeters, and its bladder now fills with urine. Nuchal translucency can still be measured until 13+6.

A single peach lies in an open palm on beige linen
Peach61 to 73 mm, the size of your baby at the end of the week

Week 13 of 40

Calculate your week
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How big is the fetus?

0102030405060708090 mm11th week33 to 4212th week44 to 5413th week54 to 7414th week74 to 87Peach at the same scale

54 to 74 mm

By the end of the week, the fetus is about the size of a peach. Its bladder fills with urine and can be seen on ultrasound as a dark circle.

Crown-rump length according to the UNSW Embryology table; the NHS gives the same values. A peach usually measures 61 to 73 mm.

This week

Your baby

  • From now on, the movements become more varied in speed and strength. You cannot feel them yet.
  • The sex organs are formed by the 14th week of pregnancy.
  • The heart continues to beat a little more slowly; by 14+0, the rate drops on average to about 156 times per minute.

Your body

  • The median hCG level is 66,676 mIU/ml; 27,832 to 210,612 is normal. More in the hCG levels chart.
  • For about half of pregnant women, nausea continues until the end of the 14th week of pregnancy.
  • Your belly gradually becomes visible; the uterus can now be felt above the pubic bone.

What matters now

  • First-trimester screening with nuchal translucency is still possible until 13+6.
  • In Germany, the Robert Koch Institute (RKI) recommends the flu vaccination for all pregnant women from the 2nd trimester, that is, from now on.
  • In Switzerland, from the 13th week of pregnancy, you no longer pay a deductible or a co-payment for maternity benefits.

Get checked right away: heavy bleeding, severe or persistent abdominal pain, fever, pain when urinating, or vomiting where you cannot keep anything down.

Gestational sacPlacentaFetus with heartbeat

On ultrasound

  1. 12+0

    Bladder should be visible in the lower abdomen by now at the latest.

  2. 12+0

    Sex was correctly identified in studies at 12 completed weeks in 75 to 99 out of 100 cases.

  3. 13+6

    Nuchal translucency can be measured up to this day, at a crown-rump length of 45 to 84 mm.

In Germany, under the prevailing interpretation of the Genetic Diagnostics Act (Gendiagnostikgesetz), the sex may only be disclosed from 14+0. In Switzerland, this is already allowed from 12+0 after an NIPT.

Frequently asked questions about the 13th week of pregnancy

Is the 13th week of pregnancy in the 1st or 2nd trimester?

In the 2nd trimester. According to the German way of counting, it begins with the 13th week of pregnancy, that is, at 12+0. Professional societies in the USA only count it from 14+0.

What is the risk of miscarriage in the 13th week of pregnancy?

In a large London study with an examination between 11+0 and 13+6, about 1 in 100 pregnancies still ended in miscarriage after that.

Can you see the sex in the 13th week of pregnancy?

Often, yes. Whether your doctor is allowed to tell you depends on the country: in Germany, under the prevailing interpretation, only from 14+0.

Which month is the 13th week of pregnancy?

The 4th month of pregnancy, in the 2nd trimester. Figures such as 12+4 weeks also belong to the 13th week of pregnancy.

Sources

Updated on 30 October 2026 · Fertilio Editorial Team