Hyperemesis gravidarum (HG): severe sickness in pregnancy

If you are being sick often and cannot keep food or drink down, tell your midwife or doctor, or contact the hospital, as soon as possible: the NHS says there is a risk of dehydration, and that your midwife or doctor can make sure you get the right treatment (NHS, "Severe vomiting in pregnancy", 2023). It advises calling your midwife or GP, or its urgent advice line, if you are vomiting and have very dark urine or have not peed in more than 8 hours, cannot keep food or fluids down for 24 hours, feel very weak, dizzy or faint when standing up, have tummy pain or a high temperature, vomit blood or have lost weight (NHS, "Vomiting and morning sickness", 2024). The box above shows the emergency and urgent numbers for where you are, and get help now lists the symptoms that need checking straight away.

What hyperemesis gravidarum is

What is hyperemesis gravidarum? ACOG calls it the most severe form of nausea and vomiting of pregnancy (ACOG, "Morning Sickness: Nausea and Vomiting of Pregnancy", 2025), and the health bodies that define it agree on that much. They differ in what they put first, as the table shows: daily life and eating and drinking in the UK, weight and dehydration in the US.

The hyperemesis gravidarum definition also differs on one test. MedlinePlus lists urine ketones among the laboratory tests done to check for dehydration (MedlinePlus, "Hyperemesis gravidarum", 2025), while the RCOG guideline says ketones in the urine are not an indicator of dehydration and should not be used to judge how severe the sickness is (RCOG, "Green-top Guideline No. 69", 2024). This guide does not choose between them; your own care team works to its own criteria.

HG in pregnancy

HG is the abbreviation the NHS uses (NHS, "Severe vomiting in pregnancy", 2023), and HG pregnancy, HG in pregnancy, HG during pregnancy, pregnancy HG and HG pregnancy sickness all mean this condition, as do hyperemesis in pregnancy and hyperemesis pregnancy. The RCOG guideline calls the whole range of sickness "nausea and vomiting of pregnancy" and says the everyday name, morning sickness, is inaccurate because symptoms happen before and after noon, and is felt by people who have it to trivialize the condition (RCOG, "Green-top Guideline No. 69", 2024).

Hyperemesis gravidarum vs morning sickness

Severe morning sickness and HG sit on one scale rather than either side of a fixed line: the RCOG guideline calls HG a severe form of nausea and vomiting of pregnancy (RCOG, "Green-top Guideline No. 69", 2024). The sources set out where they differ:

The NHS adds that HG is much worse than the normal nausea and vomiting of pregnancy, and that other conditions can cause nausea and vomiting, which a doctor needs to rule out first (NHS, "Severe vomiting in pregnancy", 2023). When does morning sickness start sets out the timing of ordinary pregnancy sickness from ACOG, the NHS and two diary studies.

How common it is

How many people have HG depends on who is counting. The NHS says the exact number is not known because some cases may go unreported, but that it is thought to be around 1 to 3 in 100 (NHS, "Severe vomiting in pregnancy", 2023). ACOG says up to 3 in 100 pregnancies (ACOG, "Morning Sickness: Nausea and Vomiting of Pregnancy", 2025), and the RCOG's patient information also says up to 3 in 100 (RCOG, "Pregnancy sickness", 2024). The RCOG guideline gives a wider range, between 0.3 and 3.6 in 100 (RCOG, "Green-top Guideline No. 69", 2024). The figures differ partly because the definitions in the first table differ, and partly because, as the NHS says, some cases are never recorded.

When HG starts, peaks and ends

When does hyperemesis gravidarum start? The sources give the start of pregnancy sickness as a whole rather than of HG alone. The RCOG says it starts early in pregnancy, usually between the 4th and 7th week (RCOG, "Pregnancy sickness", 2024), and MedlinePlus says severe nausea and vomiting may start between 4 and 8 weeks of pregnancy (MedlinePlus, "Hyperemesis gravidarum", 2025). When does HG start in pregnancy, then, has the same answer as for milder sickness; what differs is how severe it becomes.

How long does hyperemesis gravidarum last? The sources differ. MedlinePlus says severe nausea and vomiting often goes away by weeks 14 to 16, and that some women have it for the whole pregnancy (MedlinePlus, "Hyperemesis gravidarum", 2025). The NHS says that, unlike regular pregnancy sickness, HG may not get better by 16 to 20 weeks and may not clear up completely until the baby is born, although some symptoms may improve at around 20 weeks (NHS, "Severe vomiting in pregnancy", 2023). The RCOG says the symptoms usually go or improve after the birth, and advises contacting your midwife or GP if severe nausea and vomiting goes on after 20 weeks (RCOG, "Pregnancy sickness", 2024). None of the sources cited here gives a week when HG is at its worst.

The week it starts matters for another reason. The RCOG guideline defines nausea and vomiting of pregnancy as starting before 16 weeks with no other cause (RCOG, "Green-top Guideline No. 69", 2024), and the RCOG says other causes should be looked into if vomiting only starts after 16 weeks, or comes with tummy pain, pain passing urine or a history of conditions such as a stomach ulcer or gallstones (RCOG, "Pregnancy sickness", 2024). ACOG draws the line earlier: nausea and vomiting that first starts after 9 weeks, or comes with abdominal pain, fever, headache or a swollen thyroid gland, may be a sign of another condition (ACOG, "Morning Sickness: Nausea and Vomiting of Pregnancy", 2025).

The week-by-week guide covers what the NHS describes at 6 weeks pregnant, 7 weeks pregnant, 8 weeks pregnant, 9 weeks pregnant, 10 weeks pregnant, 11 weeks pregnant and 12 weeks pregnant.

Hyperemesis gravidarum symptoms and warning signs

The hyperemesis gravidarum symptoms the NHS lists are prolonged and severe nausea and vomiting; dehydration, which it describes as feeling thirsty, tired, dizzy or lightheaded, not peeing very much, and dark yellow, strong-smelling pee; and weight loss (NHS, "Severe vomiting in pregnancy", 2023). MedlinePlus adds salivating a lot more than normal, constipation, being unable to take in enough fluid or food, and signs of dehydration such as dark urine, dry skin, weakness, lightheadedness or fainting (MedlinePlus, "Hyperemesis gravidarum", 2025). The RCOG describes dehydration as feeling dry or very thirsty, becoming drowsy or unwell, or urine changing from light yellow to dark yellow or brown (RCOG, "Pregnancy sickness", 2024).

The symptoms of HG overlap with the early signs of pregnancy, of which feeling or being sick is one; early pregnancy symptoms lists the others.

When to get help

In the UK, the NHS advises seeing your GP or midwife if you have severe nausea and vomiting, because getting help early can help you avoid dehydration and weight loss (NHS, "Severe vomiting in pregnancy", 2023), and the list in the first paragraph of this guide sets out when to call. The RCOG says your GP or midwife will arrange for you to be seen in the hospital's assessment unit, which may be part of the maternity department or the gynecology unit (RCOG, "Pregnancy sickness", 2024).

In the US, ACOG advises calling your ob-gyn or other obstetric care provider for signs of dehydration: a small amount of dark urine, being unable to urinate, being unable to keep liquids down, feeling dizzy or faint when standing up, or a racing or pounding heartbeat (ACOG, "Morning Sickness: Nausea and Vomiting of Pregnancy", 2025). MedlinePlus advises contacting your provider for severe nausea and vomiting, or for signs of dehydration, being unable to keep any fluids down for more than 12 hours, lightheadedness or dizziness, blood in the vomit, abdominal pain, or losing more than 5 pounds (2.3 kilograms) (MedlinePlus, "Hyperemesis gravidarum", 2025).

What causes it

What causes hyperemesis gravidarum? Its causes are better understood than they were. The NHS says it is not known exactly what causes HG or why some women get it and others do not, and that there is evidence of a link to the changing hormones of pregnancy (NHS, "Severe vomiting in pregnancy", 2023). The RCOG says research suggests some women have worse pregnancy sickness because of genetic differences involving a hormone called GDF-15, which is made by the placenta and causes loss of appetite and nausea (RCOG, "Pregnancy sickness", 2024). MedlinePlus goes further and calls GDF15 the main cause, and says people with lower levels of the hormone before pregnancy are more likely to react to the sudden rise in early pregnancy (MedlinePlus, "Hyperemesis gravidarum", 2025).

That account rests on a 2024 study in Nature. It found that most of the GDF15 in a pregnant woman's blood comes from the fetus and placenta, that higher levels are linked to vomiting in pregnancy and to HG, and that low levels of the hormone before pregnancy, from rare and common genetic variants, increase the risk of HG. The authors describe their findings as supporting a likely causal role for GDF15, with a woman's sensitivity to it, partly set by her levels before pregnancy, a major influence on how severe the sickness is (Fejzo et al., 2024). What causes HG in pregnancy, on this evidence, is mainly a hormone from the placenta acting on a body that had little of it before pregnancy.

The sources list factors that make HG more likely. The NHS names HG in a mother or sister, and HG in a previous pregnancy (NHS, "Severe vomiting in pregnancy", 2023). ACOG lists a multiple pregnancy, nausea and vomiting in a previous pregnancy, a mother or sister who had severe nausea and vomiting in pregnancy, a history of motion sickness or migraines, and being pregnant with a female fetus (ACOG, "Morning Sickness: Nausea and Vomiting of Pregnancy", 2025). MedlinePlus adds a molar pregnancy (MedlinePlus, "Hyperemesis gravidarum", 2025). These are patterns across many pregnancies; none of them predicts HG in one.

What happens at an assessment

The RCOG describes the check-up in the UK. It may include questions about how long you have had symptoms, whether you are keeping fluids and food down, whether you have tried any medicine, whether you have lost weight quickly, any other symptoms and whether you had this in a previous pregnancy; a check of your temperature, pulse, breathing rate and blood pressure; an offer to measure your weight, and an assessment of whether you are dehydrated or undernourished; urine tests to look for infection; blood tests; and an ultrasound scan, if you have not had one yet, to check how many weeks pregnant you are and whether you are expecting twins (RCOG, "Pregnancy sickness", 2024). If you can keep some fluids down but cannot drink enough, it says you should be offered fluids through a drip in your arm over a short time.

The RCOG says admission to hospital may be advised if you are dehydrated; have severe vomiting and cannot keep any fluids or medicine down; have abnormal blood tests; have lost a lot of weight quickly; or have a medical condition such as a heart or kidney problem, diabetes, epilepsy or HIV and cannot take medicine by mouth (RCOG, "Pregnancy sickness", 2024). That list is the clearest answer the sources give on hyperemesis gravidarum and when to go to hospital.

In the US, MedlinePlus says your provider will examine you, that your blood pressure may be low and your pulse high, and that tests may include a complete blood count, electrolytes, urine ketones and your weight, with checks for liver and gut problems if needed. A pregnancy ultrasound looks for twins or more and checks for a molar pregnancy (MedlinePlus, "Hyperemesis gravidarum", 2025). If you are not sure how many weeks pregnant you are, the due date calculator works it out from your last period; the scan at an assessment checks it.

Treatment is decided with your doctor or midwife, and this guide does not cover medicines. The NHS says medicines can be used in pregnancy, including the first 12 weeks, to help with HG, and that if the sickness cannot be controlled you may need to be admitted so doctors can assess you and give the right treatment (NHS, "Severe vomiting in pregnancy", 2023).

HG, your baby and your mental health

The NHS says HG can make you feel very unwell but is unlikely to harm your baby if it is treated effectively, and that if it causes you to lose weight there is a higher chance of your baby being born smaller than expected (NHS, "Severe vomiting in pregnancy", 2023). The RCOG says there is no evidence that mild to moderate nausea and vomiting harm the baby, that severe sickness or HG may lead to a lower than expected birthweight, and that you may be offered scans to check your baby's growth (RCOG, "Pregnancy sickness", 2024). The NHS also says that because HG can cause dehydration, the risk of a blood clot is higher, although this is rare, and that treatment can be given to prevent clots if you are dehydrated and not moving about (NHS, "Severe vomiting in pregnancy", 2023).

HG can affect how you feel as much as your body. The NHS says it can affect you emotionally and physically, that without treatment it may lead to complications such as depression, and that you may feel anxious, isolated or unsure how to cope; it asks you to tell your midwife or doctor how it is affecting you (NHS, "Severe vomiting in pregnancy", 2023). The RCOG says some women become depressed and need extra support such as counseling (RCOG, "Pregnancy sickness", 2024), and MedlinePlus says HG can cause anxiety and depression that sometimes last after the pregnancy (MedlinePlus, "Hyperemesis gravidarum", 2025). In the UK, the NHS points to the charity Pregnancy Sickness Support, which can put you in touch with someone who has had HG (NHS, "Severe vomiting in pregnancy", 2023).

HG in another pregnancy

The NHS says that if you have had HG before, you are likely to get it again in another pregnancy. It suggests planning ahead, such as arranging child care so you can rest, trying what helped last time, and talking to your doctor about starting medicine early (NHS, "Severe vomiting in pregnancy", 2023). How to prevent hyperemesis gravidarum is not yet known: none of the sources cited here describes a proven way to prevent HG, and the authors of the GDF15 study say their findings suggest approaches to prevention based on how the hormone works (Fejzo et al., 2024).

This guide is information, not a diagnosis or a recommendation of treatment, and it is no substitute for your own clinician. See the medical disclaimer.