When the Pregnancy Sac Is Empty: Understanding Anembryonic Pregnancy

Early in pregnancy, a small fluid-filled sac, called the gestational sac, forms in the womb. On a scan the embryo normally appears inside it as a small thickening known as the fetal pole. In an anembryonic pregnancy the sac forms and keeps growing, but the embryo never develops, or stops so early that it is reabsorbed and cannot be seen.

Because the sac and early placental tissue can go on growing for a while, and pregnancy hormones can stay high, you may still feel pregnant until a scan shows otherwise. Miscarriage UK notes that it is most often found between about 8 and 13 weeks. It is one form of missed, or silent, miscarriage, and the likeliest cause is a chance problem in very early development, often a chromosome error, not anything you did.

Guidance from the Royal College of Obstetricians and Gynaecologists (RCOG) describes one-off early miscarriages as often caused by random chromosome changes in the pregnancy. The commonest is a trisomy, an extra copy of one chromosome, which usually arises as the egg develops and becomes more likely as the egg gets older.

In the larger study above, empty-sac pregnancies less often had triploidy (a whole extra set of chromosomes) or sex-chromosome changes, and more often had tetraploidy (two extra sets), than losses where an embryo was seen. The tetraploidy difference held when the authors rechecked some samples on uncultured tissue.

Roughly 4 in 10 empty-sac pregnancies in that study had no change the method could see. The authors suggest some of these may involve changes too small for older tests, or faults in single genes or in how genes are switched on and off.

Ordinary exercise, sex and work are not known causes. Was it something I did? covers the evidence on lifestyle, and chromosomal and genetic causes explains the main types of change.

Miscarriage UK explains that an empty sac can be left to come away naturally, which may take time while hormone levels fall, or the process can be helped with medication or surgery. Unless you need emergency care, you should usually be able to choose, although your history or local services may limit the options.

In a UK study of 92 women with an empty sac who chose to wait, about 1 in 4 miscarried naturally within a week of diagnosis, about half within two weeks and around two-thirds by the end of follow-up, which lasted up to about six weeks; the others were offered surgery. Just diagnosed compares the options.

It can, but it is not likely. Miscarriage UK describes anembryonic pregnancy as a chance event and says most women go on to have a healthy pregnancy. RCOG guidance notes that when a miscarriage is found to have a chromosome change, the outlook for the following pregnancy tends to be better than after a loss whose chromosomes were normal, comparing women of the same age.

In the UK, recurrent miscarriage means three or more losses, which is usually when the NHS starts investigations. RCOG lets doctors begin after two if they suspect an underlying cause, for example when an earlier loss had normal chromosomes. See recurrent miscarriage and preparing for your next pregnancy.

Some people feel they should not be so upset because no embryo was ever seen. You were pregnant, and may already have been picturing a future with this baby, so the grief can be just as real.

Your NHS early pregnancy unit is the place to start for repeat scans, choices about managing the miscarriage and questions about NHS tissue testing. If you are unsure about the diagnosis, you can ask the unit for a second opinion.

If you need to talk, Miscarriage UK runs a support line (0303 003 6464), and Tommy's has a free midwife-led line on 0800 0147 800.

Some people also want a further scan, a second specialist view or advice on genetic testing outside the NHS. Our team can help with these alongside your NHS care, as set out below. Whoever arranges a test for you, ask which method the laboratory uses and what you would be offered if the sample gives no result.