13 Weeks Pregnant: Baby Development

Week 13 marks the finish line of the first trimester — a milestone worth acknowledging. Your baby is now roughly the size of a lemon, and while there's still a long road of growth ahead, some of the trickiest early symptoms often start to fade from around this point.

Baby's development this week

At 13 weeks, your baby measures around 7.4cm from crown to rump and is developing fast:

Fingerprints are forming on the tiny pads of their fingers — completely unique to them.
Vocal cords are developing, though obviously there's no sound yet since the lungs aren't in use.
The intestines are relocating. Earlier in the first trimester, part of the bowel develops inside the umbilical cord because there isn't room in the abdomen; by around this week it moves into its permanent position inside the tummy.
The kidneys are working, and your baby is swallowing small amounts of amniotic fluid and passing it as urine — an important part of practising the digestive and renal systems.
Bones are hardening (ossifying), particularly in the skull and long bones, though they'll stay flexible enough for delivery until birth.
The placenta has largely taken over hormone production from the corpus luteum, which is part of why some early pregnancy symptoms start to settle around now.

Your body at 13 weeks

Reaching the end of the first trimester is genuinely significant. For many people, this is the week that:

Morning sickness noticeably improves or resolves, though timelines vary widely and it's completely normal for some nausea to persist longer.
Energy starts returning as your body adjusts to hormonal changes.
The bump becomes slightly more defined, particularly if this isn't a first pregnancy.
The urge to urinate frequently may ease slightly as the uterus rises up out of the pelvis, taking pressure off the bladder — though this can also reverse again as the uterus grows further later in pregnancy.

Looking ahead to the second trimester

The second trimester (weeks 13–27) is often described as the most comfortable stretch of pregnancy for many people. It's a good time to start thinking about your next round of antenatal appointments, including your 20-week anomaly scan, and to begin considering things like antenatal classes, maternity leave planning, and — if you haven't already — sharing your news more widely.

Frequently Asked Questions

When exactly does the first trimester end?

The first trimester is generally counted as weeks 1–12 or 1–13, depending on the source, with the second trimester beginning at week 13 or 14. There's no single universally agreed cut-off, so don't worry if different apps or resources give you slightly different week numbers — what matters clinically is your dating scan measurement, not the trimester label.

Why do intestines develop outside the body early in pregnancy?

In early pregnancy, the abdominal cavity is too small to hold the rapidly growing intestines, so a portion temporarily develops within the base of the umbilical cord — this is a completely normal part of typical development called physiological herniation. By around 11–13 weeks, the abdomen has grown enough for the intestines to move into their permanent position inside the tummy.

Will my symptoms definitely improve now the first trimester is over?

For many people, yes — nausea, fatigue and breast tenderness often ease considerably from around 13–14 weeks as hormone levels stabilise. However, this isn't universal; some people continue to feel unwell well into the second trimester, and that doesn't necessarily mean anything is wrong. If symptoms are severe or you're worried, mention it to your midwife.

This article is for informational purposes only and does not constitute medical advice. Always speak to your midwife or GP about any symptoms or concerns.

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