Week 13 closes the first trimester. hCG is falling, the placenta is fully in charge of hormone production, and for most women appetite and energy start coming back after three difficult months.
Your baby is about 75 mm and 22 grams — lemon-sized — and the body is finally growing faster than the head.
The short answer: the first trimester ends at week 13 by the usual convention. The placenta now produces all the progesterone and oestrogen needed, hCG has fallen, and nausea and fatigue typically ease. Unique fingerprints form this week, the vocal cords are complete, and the intestines — which had been partly inside the umbilical cord — have moved fully into the abdomen. Miscarriage risk drops substantially from here.
Your baby this week
The body is now growing faster than the head, so proportions start to even out — the head was about half the total length a few weeks ago and will keep shrinking as a share of it.
This is the week fingerprints form: ridges on the fingertips in a pattern that is unique and permanent. The vocal cords are complete, though there is no air to use them with for another six months.
The intestines have finished migrating. Earlier in development they grew faster than the abdominal cavity could hold and sat partly inside the base of the umbilical cord; they have now moved fully inside the abdomen, which is a normal step, not a complication.
Your body this week
The placenta is fully functioning and producing all the progesterone and oestrogen the pregnancy needs. hCG has fallen from its peak, and with it usually goes the worst of the nausea. Appetite returns — often sharply — and this is where paying attention to weight gain starts to matter, because the second trimester is when most of it happens. Our weight gain guide sets out the corridor for each starting BMI.
The uterus is growing upward and forward, and the top of it — the fundus — can now be felt just above your pubic bone. This is where a bump often becomes visible, though the timing varies enormously between women and between pregnancies.
Protein needs rise now, as tissue-building accelerates. Lean meat, beans, lentils, eggs, dairy and nuts all count.
Tests and appointments
If the 11–14 week screening has not happened, this is the last realistic week to arrange it. After 14 weeks the nuchal measurement is no longer valid, and the alternative becomes the quadruple blood test between 15 and 20 weeks — a screening test with lower detection rates than the combined test.
✅ Worth doing this week
- Increase protein as tissue building accelerates — meat, beans, lentils, eggs, dairy, nuts
- Start weighing yourself consistently, same day and conditions, as most gain happens from here
- Confirm the nuchal scan happened — after 14 weeks the window has gone
- Restart gentle exercise if the first trimester stopped you; the second is usually the easiest for it
- Moisturise the abdomen as it begins to round out and the skin stretches faster
When to call your doctor
Call the same day if: you have bleeding, severe or one-sided abdominal pain, a severe headache with visual disturbance or sudden swelling of the face and hands, fainting, a temperature above 38°C, burning on passing urine, or severe itching without a rash. From the second trimester onwards, blood pressure and protein in the urine are checked at every appointment because pre-eclampsia becomes the condition being watched for.
FAQ — 13 weeks pregnant
Is 13 weeks the end of the first trimester?
Yes, by the most common convention: the first trimester is weeks 1–13, the second 14–27 and the third 28–40. Some sources end the first trimester at 12 weeks instead, which is why you will see both. Nothing biological changes on the boundary — it is a way of dividing the pregnancy, not an event.
How big is the baby at 13 weeks?
About 75 mm crown to rump and roughly 22 grams — lemon-sized. From week 14 the measurement changes to crown-to-heel, which is why the length figure jumps sharply between this week and next. That is a change in how it is measured, not a growth spurt.
When will I start showing?
The top of the uterus can now be felt just above the pubic bone, and many women notice a definite change in shape between 13 and 16 weeks. It varies enormously: a first pregnancy usually shows later, a second earlier, and body shape and the position of the uterus both matter. Showing early or late says nothing about the baby's size.
Is the risk of miscarriage lower now?
Substantially. The great majority of miscarriages happen in the first trimester, and once a heartbeat has been seen at a 12-week scan the risk for the rest of the pregnancy drops to a low single-digit percentage. That is the practical reason many people choose to share the news after this scan — though there is no obligation to wait, and telling people early means having support if something does go wrong.
Week 13 at a glance
Thirteen weeks ends the first trimester. Your baby is about 75 mm and 22 grams, with unique fingerprints forming, complete vocal cords, and intestines that have finished migrating from the umbilical cord into the abdomen. The body is at last growing faster than the head. The placenta produces all the hormones the pregnancy needs, hCG has fallen from its peak, and nausea and exhaustion usually ease — appetite often returns sharply, which is why weight tracking starts to matter now. The fundus is palpable above the pubic bone, and miscarriage risk drops substantially from this point.
Inside Baby Novum: the weight tracker plots your gain against the WHO corridor for your pre-pregnancy BMI rather than a single target number, which matters most in the second trimester when the majority of gain happens.