About & sources

Baby Week Atlas explains pregnancy one week at a time in plain language. It is general information, not medical advice, and doesn’t replace your midwife or doctor.

How weeks are counted

“Week 12” means 12 weeks 0 days to 12 weeks 6 days, counted from the first day of your last period. That is the same convention clinicians use.

Sizes

Lengths follow the NHS week-by-week guide: head to bottom up to week 19, and head to heel from week 20. Weights from week 20 are the 50th percentile of the WHO Fetal Growth Charts. Weights before week 20 are approximate, and close to Mayo Clinic’s figures where we compared them, so treat them as a rough guide. Real babies vary a lot and still grow healthily.

The images

The baby images were made with an AI image generator, from prompts describing each stage of development, then adjusted and retouched so the series stays consistent from week to week. They are educational illustrations, not photographs or ultrasound images. No doctor, midwife or medical illustrator has checked them, and some details are simplified or approximate.

Weeks 1–4 show microscopic stages: there is no embryo in gestational weeks 1–2. From week 5, the main viewer places the baby inside a womb shown in cross-section. The womb is drawn by a computer program, so its size, wall thickness and the placenta’s position are approximate, and the amniotic sac isn’t shown. Real placentas can attach in different places.

The baby keeps nearly the same pose so you can compare weeks, but real babies move around a lot. The images switch to head-down at week 34, while most babies are head-down by about 36 weeks. The eyes stay closed in every image, even though babies can open them from around week 26, and vernix, the waxy coating on the skin, isn’t shown.

The main viewer uses a smooth relative-growth scale, so the baby gets taller each week without the earliest embryo disappearing. The “real size” comparison next to an adult hand is the to-scale view.

How we check this

Every medical claim is meant to trace back to a named source, and each week and myth page lists its references. We use, in order: official health bodies (NHS, WHO, CDC, FDA, MedlinePlus), then published studies, which we check on PubMed against their abstracts.

Some things you should know. No doctor or midwife has reviewed this site. The content was written and checked with the help of an AI tool, working from the sources listed. A few references, mostly local NHS trust leaflets, were checked through search summaries rather than the full page. A few general explanations of how the body develops come from standard embryology and have no individual citation. Where we couldn’t find a study testing a claim, the page says so.

Because of that, please treat this as a guide to what the sources say, not medical advice. If anything here differs from what your midwife or doctor tells you, follow them.

Main references